I am a big fan of teaching infants sign language.  What a wonderful tool for communicating before baby learns to talk!  It saves baby from so much frustration; and isn't it nicer to see a sign than to witness whining, banging, and pointing?  Everyone needs tools to communicate - and those tools must be taught.  I bought a 
signing book to use with Drew when he was about 7 months old.  Unfortunately I didn't learn (or teach) as many signs as I would've liked, but he did learn "milk" "more" "please" "airplane" and "all done."  There is a show on PBS, 
Signing Time, that others might also use as a teaching tool.  As minimal as our signing experience has been, I found it valuable.  I plan to do it again with baby # 2 and maybe even take 
a class! 
Drew began signing before his first birthday.  I can't remember exactly when it happened, but I remember encouraging him to sign "milk" before he got a bottle.  It worked.  And he quickly learned to express that he wanted more (or not) when eating solids.  Later on I felt some 
manners were important and taught him to sign "please" which he quickly learned would get him what he wanted :)
Now Drew is 19 months and has an incredible vocabulary.  He can say, or repeat, nearly anything.  It's amazing!  The irony is that he will not say the words: more, milk, or please.  If I ask him to "Say Milk" he simply signs it.  He continues to sign those three words constantly.  I've heard the theory that children that sign often speak later, and I wondered if it was true.  I think if you have a means to communicate you might be less inspired to learn to speak.  I'm not sure.  I do think it's interesting that Drew still refuses to speak those three words.  It doesn't matter much to me
 how he communicates as that he 
can communicate.