Finally. Skype’s latest iPhone update (2.0.0) allows you to make Skype calls over 3G. According to the app blurb, 3G Skype-to-Skype calls will be free until the end of August (data charges will still apply), after that you will get charged.
A variety of other apps, including Fring, have allowed you to make VoIP calls over 3G for some time now. It’s been rumoured that the iPhone 4G will have a forward facing camera so we hope that Skype introduces video calling soon too.