DD was in a home daycare from about 6 months to nearly 3 years when I was working part time. She had some structured learning time there so it was much like a small preschool environment from a very early age. When I decided to SAH full time, I didn't want to take that social environment away from her because she is a very social kiddo and knew I wouldn't be good at having structured learning time with her.
We put her in montessori because based on the time of the year we started looking, my options for places (space availability) were limited, and it was the best place out of all the ones I visited. She goes to school 5 mornings/week. I do keep her home from time to time just to have more days with her because going every day just feels like a lot sometimes. However, we are now really on board with the montessori philosophy and any of the programs I have looked at are 5 days/week. I have to admit, sometimes, I wish I would have kept her home this last year and waited until this coming fall to put her in preschool. She will be 4 at the end of Sept and will start K in 2011.
What are your thoughts on preschool? Will you send your LO? At what age do you plan to start?

Re: Your thoughts on Preschool
All of the "regular" preschools in our area won't take kids under 3. DD will be 2 in April, so we are going to try a co-op preschool starting in September because I think she will be ready for more structured learning (they can start at 2 years old). It's only 2 days per week for 2 hours each day, and parental involvement is required. I don't think early preschool is absolutely necessary for every kid, but I think DD will like it. We do a lot of activites and fun stuff every day (playdates, childrens museum, park, hanging at home) but nothing really structured like school.
We will keep her in the co-op if we like it or switch her to a mainstream preschool at age 3. We are planning on private school from K-12.
I can understand your hesitation with "loosing" her to school... I'm so not ready!!! ;(
Preschool is for children 2 years 9 months and older here in CA and just for me personally, there's no way I'd want to send Emily to a 5-morning per week program at that age. I have loved having her home with me up through now (she's 3 1/2 years old!) and am only just now enrolling her in a 2-morning per week Preschool program with our local Parks & Rec. Department. While this program is a real Preschool, it's more fun and play and just a great way for her to be with several of the kids we've known for years as they are all enrolled and for me to have just a few hours each week to get things done... alone!
Preschool actually starts for us this fall, 5-mornings per week. Emily will be 4 years old.
eclaire 9.10.06 diggy 6.2.11
Matthias will be three in May and will be going to preschool two mornings a week starting this fall. Then the following year he'll go three mornings a week and then the following year, he'll be in kindergarten five mornings a week (unless the school we want changes the kindergarten to a full day program).
Ezra will follow the same schedule. Now take this with a grain of salt, because the boys will be going to an in-home daycare starting in June. But it is a very small daycare owned by a friend of ours, so it will almost be like a SAH situation.
Baby # 2 edd 11/26/08 - Ezra Jacob born 11/29/08, 9 lbs 6 oz., 21 3/4 in
Baby #3 edd 05/04/13 - Titus Jude born 05/01/13, 9 lb 5 oz. 21.5 in
In my area preschool starts at 3 y.o.(2 days/wk, 2.5 hour sessions), 4 y.o. preschool is 3 days/wk, 2.5 hour sessions. If people want to send their kids earlier than 3 then they can seek out a daycare setting or Montessori starts at 2.5 y.o.
With my oldest he had a late birthday so he only went to one year of preschool prior to entering Kindergarten, but my daughter has a spring birthday so she attended both 3 & 4 y.o. preschool. If your Montessori is only in the morning I don't see the problem with sending her 5 days/wk. You still have every afternoon to go on an outing or just spend time with her at home.
our area has preschool for 3 year olds. our daughter is a great daycare right now-2 days per week and learning tons. it is an in-home daycare and she does a curriculum etc. this next year our daycare will be closed so we are going to keep our daughter home and i will continue to work with her on abcs, numbers, coloring and all the stuff. when she is 4 we'll probably do preschool a few days per week to get the rule following from others and school like structure started.
i just don't think a 3 year old is ready for preschool and for what we would pay she already knows what they would teach her. and socialization we will continue to get through mom's group and playdates etc. besides 3 year old preschool and even 4 year preschool is only a couple hours and for the price just not worth it.
I have read that socialization skills start developing between 2 and 3; before that, playing with others is not necessary, although of course more outgoing kids like to hang out with other kids.
Right now, DD goes to two classes a week, plus the library, plus play dates, so I feel like she's getting enough interaction with LOs her age. Since she's a bit of a loner, I think she'll need to be around others in a more structured way around 3, to prepare her for kindergarten, so we'll be doing 1/2 day pre-school then. This also will give me 3 hours a day to do my WAH in peace and quiet.
Personally, I want her to have as much free play as possible. I'm not into making every.single.thing DD does about learning. She will learn the things she needs to learn when she's ready. I just want her to have as much fun as possible while she's a kid. But she does LOVE music, so we'll try to find a place that does daily music time.
I will be the odd man out and say and an educator I think preschool is very important. It has very little to do with learning and a lot more to do with following the rules, schedules and listening to an adult that is not a parent. It is also a necessity for children to socialize without Mommy around.
DD plays very very differently when I am around and when I am not. She goes to preschool 3 days a week from 8:30-2:30, I miss her when she is not here, but it is not about me.
Also to adress the pp about free play. Any good preschool at this age is a play based preschool with tons of free play time, this is afterall how kids learn at this age.
DD started MDO at 18 months and so will DS. She started preschool at 3 and so will DS.
I think a year of preschool is very important before a child goes to kinder. This is JMO, but based on what i have seen as an educator I think it is very valid.
DD has a mid sept birthday, so she will start kindergarten a few weeks before she turns 6 (2014-2015 school year). Thus, our plan is full day preschool the previous year (2013-2014) and probably spanish immersion. And a very part time spanish immersion preK for 2012-2013. I don't envision her in preschool programs before that. And mostly that has to do with her birthday. I think 2 years of some sort of preK program before kinder is plenty, so if her birthday were different and she wasn't going to be the oldest in her class, we would start the PT preK earlier.
I think preschool programs are worthwhile and important, but I don't think 3+ years of preschool is necessary. I am sticking with the idea that kindergarten gets kids ready for a life of school schedules and routines. And that preschool is a kindergarten readiness opportunity. Prior to that, I get a little crazy that we are all pushing our kids into a system that isn't necessarily a great system, at an unnecessarily early age.
We'll see if my ideal plan actually really works out for us 
 
In this crazy area, down the road from an ivy league college and filled with executives commuting to NYC and Philly, you would be amazed (or, at least, I was) at how many pre-schools advertise that they start reading, math, even history, at 3 years old. Ai-yi-yi! I've even talked with a mom who told me she specifically was looking at academic preparation for her 2 year old! Parents here are super, super competitive, and I want to keep DD as innocent of those expectations for as long as possible.
I do want DD to learn all the things you mentioned. I'm sure they introduce these concepts in a fun, kid-oriented way. It's the parents, and how their expectations shape how the teachers teach, and how they deal with the more "free spirited" kids, that I'm worried about.
Kate starts real preschool this fall, at 3 yrs 4 months old. She will go 5 days. I would have preferred 3 days, but the co-op that we fell in love with only offers a 5 day option. I think she will thrive. She went to a 2 year old playschool for 2 mornings a week this year and has really learned so much. She is motor-delayed and I think it is really helpful for her to see other kids playing on the playground equipment, etc. It's different and more comfortable for her when it is the same kids, her friends, every day than just me taking her to the playground. She's also able to learn rules, listening to another adult that isn't me, etc. I think it's incredibly valuable.
FWIW, I live in a very competitive area, too, and we were able to find a low key, play-based co-op preschool. They are out there! I know it seems like every parent is in a rat race, but it's really not true (and I went to the Ivy League college the PP is referring to, so I know lots of competitive people/parents
I think preschool is important, but I can't imagine sending my 2.5-3YO 5 days a week. That's way too much for me to do.
We have taken a gradual approach. If I had only 1 child, then we would have waited until this year to send DD#1 to preschool (she misses the school cutoff) and she would have had 2 years of preschool before K. Because I have 3 kids, and no help around, it's hard to have 3 small kids at home all day long. The girls love preschool.
Each of my kids will have 3 years of preschool. First year = 2 half days. Second year = 3 half days and the year before K (third year) = 5 half days. Our K is full day, so I want to have an easier transition for my kids. I think 5 half days to 5 full day K will be easier than 3 half days to 5 full day K.
I do think the idea of "interviewing" a child for a preschool is weird. I can't imagine what that process is like for a 3 or 4 year old. I would definitely have not even considered a preschool like that. It's competitive in our area, and you have to sign up early, but mostly everyone gets their first choice.
I thought about sending him at 2 1/2 in Sept- there are some places that will take kids that aren't potty trained yet (if he isn't) But I changed my mind. I will send him at the normal time, the following Sept- when he'll be 3 1/2, to the 3 yr old preschool at our church. It is 2 days a week for 2 hours in the morning. Then the 4 yr old pre-K goes to 3 days a week for 2 hours.
I decided we'll stick with what we do now. Our town has a playgroup on Mondays and Friday for one hour and then we do story time at the library for 1 hour on Weds. So thats 3 days a week we do something "semi-structured" with socialization. Then this summer we plan do be a the town pool a lot- which I think is a great place for him to play and make friends.
Those play groups and story time cost $75 total for the fall, winter and sprig sessions. Which is also a lot cheaper than a preschool.
Similar to pps, where I live, preschool starts at 3, so there's a maximum of 2 years of preschool before they start kindergarten (unless you decide to hold them back, of course).
I will send my 3 yo for 2 afternoons a week (2 hours), 3 afternoons a week when she's 4.
ETA: I don't think preschool is necesary at all. However, I think DD will enjoy it.
We started DS in a 2-day/wk program at 20 months. It's a long day (5.5 hours) but he's done so well!
Our biggest motivation for sending him to the program was to interact with other little ones. We live in an area without a lot of other young families. I also really like the time alone to get things done around the house as well as take care of my volunteer commitments.
He'll be going 3-days/week next fall (2010/2011 school year). We plan to keep him in the program until he enters Kindergarten in Fall of 2013.