We at fiddme are really excited about the iPhone 4. We just got ours (it started selling in Israel only today) and we started playing with it, we noticed that you can fool your iPhone to think it is connected to a wifi while it is really connected to the 3g and here is how:
1. Enable Internet Tethering on your iPhone (Settings -> General -> Internet Tethering). This might not be available in all countries.
2. Connect your mac/pc to the iPhone (I used USB but it can be done using a bluetooth).
3. Create a wifi network on your mac/pc. At that point you should see the blue bar on your iPhone.
4. Share your internet connection from USB to Airport (on mac). On a pc it’s almost the same.
5. Now here is the trick – Connect your iPhone to that new WiFi network you just created.
Voula… Have fun…