If you want to go traditional, you can always put "Welcome Baby [Last Name]" if they don't know the sex of the baby or haven't figured out a name. I think my mom's cake (back in the 80's) had "Welcome Little One" with sugar baby booties on it. I saw a picture of it the other day, and it was actually pretty cute.
Re: Baby Shower Cake
Mine said "Welcome Baby Charlotte".
I think you can put Congratulations <insert mom's name or mom and dad's names>
I like this idea. Or if you don't want to do that you could do a like "See you in November baby!" or something....I don't know.
"Eat Me."
--my personal dream cake
This.
How about "Congratulations".
Totally genius, I know.
This cake is always a hit at a baby shower: