Apple’s adamant that you should use their FaceTime video chat software over WiFi, and even have some software checks to make sure you’re not trying to pump your pixellated mug out over 3G. It appears, though, that the iPhone 4 isn’t particularly mindful if you’re connect to 3G somewhere down the pipe: if you want to use FaceTime on an iPhone 4 over 3G, it’s just a Personal Hotspot away.
Basically, if you have iOS 4.3 installed on an iPhone 4 and use another iPhone 4 to connect to it, you can can use FaceTime over 3G. It’s a pretty simple trick: since Personal Hotspot broadcasts itself as a typical WiFi hotspot, FaceTime just doesn’t recognize that it’s nit tethered to a landline internet connection.
I doubt that this is a practical application of Personal Hotspot for most people, but it implies that users will be able to get around quite a few apps worth WiFi-only limitations with Personal Hotspot.