Truphone on the iPhone
I figured the folks at Truphone would be first with a VoIP over WiFi calling app available through the App Store. Sure enough, they are, or at least among them!
I don't have an iPhone-first or second edition, so I can't really comment on how well this application works or doesn't. It's a nice idea whose time has come, no doubt! Meanwhile, if I need to use Truphone, I've got plenty of Nokia's to install it on!