While initially having our speech disordered oldest son learn sign language seemed to stop all progress in his efforts to speak (he went from a dozen mono-syllabic poorly inunciated grunts to just using 70 sign words, in one summer)... in the end it was worth it to give him a method of communication.
It was also important to speak to this
hearing child as we used sign language. Also, speaking as one signs tends to slow down the adult's speech, making it easier for the child to understand the adult and the form of words (think of when you listen to a language instruction tape of (for instance) French, versus listening to a native speaker in a normal conversation... which one would be easier to distinguish to individual words?).
Because of early diagnosis and early intervention which included an incredible team in a special ed. preschool combined with 8 years of speech therapy he is doing quite well (though you have to be patient when he speaks because it is slow). The use of sign language reduced frustration and anxiety (there is a scene in "Mr. Holland's Opus" where the mother is upset because her deaf son cannot tell her what he wants in the kitchen... that scene played out in our house more than once).
This has a good explanation of how sign language works with kids with a certain motor disability:
http://www.apraxia-kids.org/topics/sign.html
Plus the first article in this:
http://www.apraxia-kids.org/talk/newsletterseptember_october01.html
By the way --- even with the other "normal" kids it has been handy to have a basic sign language vocabulary. This has been helpful to communicate in crowds, when I'm on the phone, noisy areas and other places. I often sign to them when they are across the pool... usually things like "sit", "look there", "listen", "no", "yes", and a particular favorite: "STOP!".
My brother got a kick one time while we were at a local mall... #2 child wanted to go into the ice-cream shop and I signed "no". He signed "yes" back... so I signed "no" --- we had a nice quiet arguement