edit: Although the identifying info from those screenshots is scrubbed, the comments say the R-URI is basically user@ip:port. This is interesting: there's no SIP proxy in between. No wonder it only works over wifi.
Unfortunately I see "Bandwidth Limit Exceeded" when I go to that page but your comment makes me wonder whether they are doing SIP URI discovery via DNS-SD[1] given their penchant for DNS-SD.
[1] http://tools.ietf.org/html/draft-lee-sip-dns-sd-uri-03 (ignore the stuff about mDNS and substitute Wide-area Bonjour)
http://www.packetstan.com/2010/07/special-look-face-time-par...
“The voice call ends as soon as the FaceTime call connects... the FaceTime call is over Wi-Fi so does not use carrier minutes.”
It's considerably more secure than an open wifi network.
Cue first FaceTime scandal in 3, 2...
“Only the call part is Vanilla SIP. The procedure for registering a Facetime user into their servers etc. is all non-SIP, encrypted/ciphered.”