Hm! I tried Facetime when I was on DL out of MSP (before losing signal over Canadia) with no success, but that could have also been because no one I knew with an iPhone 4 was near wifi at the time.

Also, SIP over VPN works OK, though quality is marginal (EVDO Rev. A upload isn't super great), and apparently, Gogo's handoffs between cell towers are not of the soft or softer variety. I tried to join in a meeting at work via phone, but my call kept dropping every minute or two for no reason, after which I could successfully make another call for another minute or two.