For those of you interested in security and privacy, the Apple FaceTime stream is not encrypted (or so it seems).
Connection to the XMPP server seems to be encrypted but communication stream between calling parties don't seem to be.
More information in a three parts blog series: Special look FaceTime (part 1: introduction), Special look FaceTime (part 2: SIP and Data Streams), Special look FaceTime (part 3: Call Connection Initialization).