I don't know of anyone who's done it yet... and despite the number of GMail users, I'm not sure enough of them are using Google Talk on a consistent and regular enough basis to make it worth while. Correct me if I'm wrong, but you can't run Google Talk as a separate application that you can Alt-Tab to and from like ICQ, AIM, or MSN Messenger.
Having said that, given Google's penchant for comparatively open API's, if you found the right documentation on their site it probably wouldn't be that hard to add it in.<