Apple's just about finished rolling out Skype for iPhone, and now you can go download the VoIP app for themselves.
Our quick test showed that voice quality is quite good—definitely on par or better than actual 3G calling over AT&T's network—and connects rather quickly. It's unfortunate that 2.2.1 doesn't have background notifications, so there's no way somebody could Skype you unless you had the app open all the time.
What's also interesting is that voice calling only works when you're on Wi-Fi in the 2.2.1 firmware, popping up a restriction message if you're on 3G, but 9to5Mac says it works just fine in the 3.0 firmware. Not sure whether that's because the 3.0 developer's beta is less locked down, but it's one pretty good reason to risk the upgrade. [App Store]