[alttag]

  > I've heard that it can lead to less frustration when the
  > child has a means to communicate with the parents.

Absolutely. Our youngest kids learned a small handful of signs (five or so) before they could speak, and it made things much easier. "More", "milk" (was hard to teach, but when it stuck it was great), "banana", and few others. Someone else here said "done" was useful, and I wish we would have done that one.