Once Apple began allowing VoIP over 3G we can now expect this functionality to apper in real iPhone applications. Fring already implemented it. Skype has announced via it's blog that a new version of Skype app for iPhone will support 3G calling:

You may have seen other apps offering calls over 3G, but we’re holding ours back for a little bit longer. Why? So that we can give you the very best audio quality we can. When our 3G-capable Skype for iPhone app is released, it’ll let you make calls in wideband audio, giving you greater clarity and fidelity – because that’s what you expect from Skype.

Watch the video for more details: