I'm wondering if anyone has tried this idea out...

I have noticed that if my iPhone is on 3G and Wifi that when I tether it to my computer the connection goes over 3G and not over the Wifi link I have. This to say, the iPhone interface will not use the same connection as the tethering. Tethering will always use 3G (or edge yuck).

Connect your iPhone to your computer and set it up to tether over 3G. Then, create an AdHoc network between your computer and the iPhone. This way, your iPhone will use the Wifi for it's gateway.

Then, see if you can use apps like Skype or a SIP phone or try and download an app over 10mb from the app store or something big from iTunes. This should all work without a jailbreak in theory but I haven't had the chance to try it yet.