As Connexion by Boeing moves closer to its big 2004 rollout on many airlines, voice over IP (VoIP) looks more likely, too: Connexion has chosen its rugged, in the air AP vendor, and is talking vaguely about how VoIP might emerge on planes. They might also deploy a low-power cellular transmitter that would use the satellite link for backhaul.
The first-generation AP on planes, by the way, is 802.11b only -- probably due to regulatory certification issues. The next-generation system will be a/b/g.