My 2.5 year old son uses it. He has a speech delay so it's really come in handy. He's gotten out of most of it since his verbal words have increased but there are a few signs he still does because he can't say the word.
We taught my first daughter sign language from about 6 months until 12 or 18 months (she could speak very well for her age by this time). I think she signed for the first time around 9 months. For us, she talked sooner than any children I knew at the time. It was great. We always said the words too and she would try to make the sound as well. It is sort of like training them to communicate. For example, if you sign "more" and they reciprocate then you give them "more" and they learn that if they tell us something (signing or verbally) we will respond/help them. I strongly recommend "My mealtime" or "My pets" from the "A Baby Signs" DVD line (find on amazon for $10). Even if your child never signs, I bet they will be mesmerized (and quiet) for the 20 minutes it is on. They have monkey sock puppets that my daughter could not take her eyes off of. Just think, it could make that shower possible... well, it worked for me anyway.
My 5 year old who has Down Syndrome uses sign language as his main form of communication right now. What is funny is my 2 year old is picking up on it so quick and we haven't worked with him at all, signing wise. I talked with my son's speech therapist and she said that children exposed to dual languages, be it Spanish and English or sign language and English, will start to talk a little later and not to worry. She was correct in our case little by little the youngest is starting to talk but in the mean time the signing is a great help for both of them to communicate.
Any one else teach their infant or toddler sign language? We worked with DD while she was an infant but she never really picked it up until now. I am so excited right now that she's really picking it up!