As an illustration of the use of sign language synthesis this video was made for the Visicast project:

(More videos are available on the Visicast site)

It is a combination of speech recognition, automatic translation and sign language synthesis. The avatar is Tessa. Unfortunately, it does not work the other way. So, if the signer needs to ask a question it gets a bit difficult. But then again, it is better than nothing. As these things go, it is but a demo video. I believe no such system exists at the moment.