At a recent Q&A session conducted by Apple executives, it was revealed that VoIP would be allowed on the iTunes App Store so long as the applications only ran over Wifi, not cellular networks (such as EDGE). Allowing VoIP to run over EDGE connections would directly compete with AT&T's voice plans, but even Wifi support for VoIP could be very useful.
This means that popular internet telephone applications such as Skype could be easily ported to the iPhone.
Apple has unveiled its software development kit for its iPhone. The SDK dramatically expands what business users and consumers can do with their mobile devices. It’s now surely only a matter of time before someone creates the VoIP program.