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VOIP over WiFi on the iPhone. This is on a jailbroken iPhone (hacked to allow 3rd party apps.) Definitely not ready for prime time, but at least it's a start. They got Skype working well although with a little delay (~2 seconds.) SkypeOut was even more flakey, but with a few successes. So at least it seems like the feasibility has been established.
After June, when 3rd party apps are officially allowed development of stuff like this should be much easier. I predict it will be usable pretty soon after that. But the deal breaker might be the "no background apps" rule from Apple. If you can't leave Skype running in the background then it's not as useful. If someone wanted to call you they'd have to text or email you to turn on the app so you could receive the call! Still, even having just outgoing calls would be useful.