The anonymity feature (assuming you want to connect people together with FaceTime) will be difficult as your phone number is shared with the person you FaceTime call. I suppose it's possible that a proxy could be used, but I have yet to take a look at the protocol specs (if they're even available).
Yeah, having never used an iPhone 4 or FaceTime I was unsure of how it all worked. Given that it's going to be a standardized protocol, a FaceTime server would be necessary even if phone numbers weren't exposed, as you'd want to manage who can connect to who (and possibly for how long).
You could even monetize by interspersing ads into the video stream.
In fact that could be a better solution - that way you could avoid building an application and locking yourself to iPhone 4 users, opening your web service to other FaceTime clients.
