Just found the slide listing the "standards" FaceTime uses [1]: H.264, AAC, SIP, STUN, TURN, ICE, RTP and SRTP. From the list, the only thing left for someone else to integrate with FT would be friend management and presence (online status). Something like Jabber could probably handle this bit pretty well.
I remember the facetime announcement including that they were going to publish it as an open standard, and I'm not the only one:
"The company said it plans to make FaceTime an open industry standard, potentially allowing
communication with other devices."
-- http://www.appleinsider.com/articles/10/06/07/apple_announces_open_standard_facetime_video_chat_for_iphone_4.html
and it's not just AppleInsider - that's just the first non-wikipedia hit for the search "facetime open standard". That said, they never (to my knowledge) delivered, which is a shame. At least now there's something, though it's a pity that FT users will likely be left out.
