Nobody knew if Apple was going to approve 3rd-party apps that use the front facing camera for video calls on the iPhone 4, but now we do. As Fring, just updated it's iPhone app to support video calling! But the great thing about this is that it that it works over 3G. It is a bit more complicated than Apple's Facetime service, which requires WiFi to work but is very simple and intuitive. But it Fring does work over 3G, which is a Plus.
Download the app Here