Ofcourse it's compatible. I was talking about Jabber, the technology.
It's the same thing as saying this forum isn't compatible because it only accepts avatars for users registered on this forum.
It's compatible because of the software used. It's an already existing thing. They could have chosen to be annoying and develop a protocol that is incompatible with MSN, ICQ, Jabber, AIM, YIM, etc... and a completely new proprietary protocol that requires their google talk program exclusively.