FaceTime video chat over cellular connection won't be possible on the iPhone 4 or iPad 2
FaceTime over a cellular network requires iPhone 4S or iPad (3rd generation) with cellular data capability. Carrier data charges may apply. FaceTime is not available in all countries."The iOS native video chat app seemingly won't work over a cellular connection on anything older than Apple's last phone and tablet: "
That's a citation from Apple's website which got a bit lost in the hullabaloo the other day. Now it's anyone's guess why would Apple neuter video chat over 3G/4G networks on its older phones and tablets. It might have something to do with performance, but, oh well, you can always hit the App Store for something like Fring and the like.