I can't say if signing would make a child smarter (as anothe poster said, then deaf children and children of the deaf would be smarter and I'm not sure I've any evidence of that). However, I think it does help with communication. My daughter is 15 months and know about 4 dozen sign now and loves to watch Signing Times on channel 13. She loves to identify things likes to use the signs for things she can't say yet. I also can't say if it's going to help with the terrible two's or not since we're not there yet, but I can see my daughter is very happy when she can communicate her needs and wants to us (even when she doesn't get what she wants, she knows I understand her because I repeat what she's asking for - "oh, you want to go to the park, well, we can't right now, we have to do xyz... blah, blah, blah" - she looks disappointed but we are talking and working it out instead of her trying to let me know what she wants and getting frustrated because I just don't get it.
Plus, it was really fun to see the light come on when she finally "got it" (around 9mo) that she could tell us stuff with her hands. She went from 4 signs (eat, more, bath & diaper change) to about 2 dozen in about two weeks time and she would just do a full-body excited giggle when she learned a new sign (it was pretty easy to tell when she got a new one back then).
Anyway, I guess that it couldn't hurt (just like teaching any other language at this point won't hurt their primary language) but gives them an additional way to communicate and the idea that there is more than one way to come to an understanding.
Good luck w/whichever you decide.
P.
Oh, and btw, Signing Times is only on Sunday morning at 6:30am... if we didn't have a dvr, we'd never see it!