Plenty of other VoIP apps have managed to work in this functionality since Apple / AT&T started allowing it last year, but the official Skype app has been a notable holdout. Now, two months after Skype started doing 3G calls on Android with Verizon, there's a 2.0 version of the app for iPhone that brings voice calls over 3G at last. Mobile iPhone calls are free until August, after which you'll need a "mobile subscription." We're testing out the app as we write this and it seems to work about as well any other 3G VoIP app we've tried: passable, not revolutionary.
[Thanks to everyone who sent this in]