Accounts are tied to the email address on your Apple ID. Interesting way of getting around having to create another IM account.
Deleted comment
Android 3.0 (Gingerbread) is rumored to gain support for the same protocol used in the desktop version of Google Talk: Jingle. Jingle sounds a lot like FaceTime, I wouldn't be surprised if the two are compatible.
http://en.wikipedia.org/wiki/Jingle_%28protocol%29
No word from Apple so far, but I recall them saying that they would share the code with whoever wants to implement it, which isn't quite the same as open sourcing it.
Open-sourcing the protocol doesn't solve the integration issue, and Apple hasn't said anything publicly about letting third parties join this particular sandbox.