My son had one word at his 15 mo appt. I was worried, But he did use sign language a lot, his receptive language was great (understanding what we say to him), and he was babbling a lot. His ped wasn't worried but did say that if he didn't have any more words at his 18 mo we could talk about having an evaluation. He is 20 mo now and goes through a week of saying a word then drops it. He has picked up a ton more signs and he is very communicative, and I'm not worried any more. It's worth talking to your ped but from everything I heard from other parents when I was freaking out, and my dr, he sounds to be well within the range of normal.
I know it's stressful though! His best friend is 5 weeks older and has been speaking in full sentences for months now!
Re: 17 month old now refusing to talk?