During the Q&A Session, Apple indicated that they would allow Voice Over IP (VOIP) applications on the iTunes App Store so long as the applications only ran over Wifi, not cellular networks (such as EDGE).
This means that popular internet telephone applications such as Skype could be easily ported to the iPhone. 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.