VoIP Heads For The iPhone?

What would Apple do if somebody came up with a VoIP program for the iPhone? Chances are, folks will prevent it from making calls over a cell network in order to afford AT&T some much needed protection. However, the story is very different if you happen to be in a WiFi hotspot, as you will then be able to save on those precious cell minutes. In a nutshell, a VoIP program for the iPhone will essentially offer what T-Mobile users have been enjoying thus far with the @Home program, albeit minus the monthly subscription fee. Will someone write a VoIP program for the iPhone already?