VoIP over 3G networks now available on the iPhone with iCall
VoIP on the iPhone has quite a turbulent past, but they eventually reached a compromise - VoIP apps are allowed but only through Wi-Fi. But that was quite limiting - VoIP apps usually offer free calls between two users running the app, but the killer feature was the cheap international calls and if you're limited to using Wi-Fi only, then you're limited to only a few locations in which you can make and receive such calls.
But Apple has lifted the Wi-Fi-only restriction through a change in the iPhone SDK and there's already an app available that takes advantage of that. It's called the iCall and it's available for download now.
Here's a short video that demonstrates the iCall doing an international call, with an interface that looks almost exactly like the regular call interface on the iPhone.
International call rates never exceed 6 cents a minute for each of the 50 countries included. Here's a more detailed map with the costs for each country.