Most AS children are fully-capable speakers - my own sons are bilingual and fluid in either spoken language.  For my son (who is now 5) with AS, I've found that teaching ASL has been a very effective way to improve his overall communication skills and also to help him grow his vocabulary.  

Over the past few years, AS has been able to convey ideas and messages that he may not know how to say otherwise - or, when he goes on a nonverbal tantrum over something infinitely minor, ASL is an enabler for him - he can still talk to me without having to say anything.

Teaching AS children ASL is a win-win, even if their speech and grammar is perfect.  It's such a visual and attention-requiring language mechanic that it helps AS children with some of their underlying weaknesses - like making eye-contact and staying focused in a conversation.  Especially with younger children, in my experience their sense of accomplishment is so high that they feel victory when they are able to communicate a proper sentence in ASL.  It helps them to enforce some of their social deficiencies - but, the key is that you parents need to tune in as well.

I used the Signing Time video series by Rachel de Azevedo Coleman.  It's targeted towards children (that can hear) and is a music-based presentation.

Are there other parents doing this?