Apple is going to allow VoIP clients on the iPhone using it's new SDK.

More specifically, Jobs stated that, as long as an application is not using the mobile carrier's network, VoIP functionality in applications will be allowed. The exact wording from the iPhone SDK agreement is that "If an Application requires or will have access to the carrier network, then additionally such Application: ... May not have Voice over Internet Protocol (VoIP) functionality."

The limitations on VoIP over cellular is probably a requirement from AT&T and is part of their contract. However I would also expect the developers to quickly work their way around this (the rub of course is the App Store).
However expect to see Skype and Gizmo Project among others to quickly get their clients out. Skype on the iPhone would make a great product to use overseas to get around ridiculous roaming charges.
The biggest benefit of the iPhone SDK is that it ushers the mobile application space into a whole new world. The App Store will revolutionist the delivery of mobile apps the same way iTunes revolutionized the music delivery world.
App Store should not be underestimated in it's impact.