Well this happened with my first iPhone 4 on ios4 and now it's happening to my new ios4.0.2 iPhone 4. The phone will become completely unresponsive, there is no swipe or selection on the home screen, the home button or power does nothing. The phone will still sleep with a black screen which will not wake up. This does not seem to sort itself out. The only thing you can do is a reset by hosing power and home.
The only thing I can think of is perhaps it is to domwith the 3GS backup I used in iTunes. But with each iPhone 4 I used a different backup file.
Quite a few others are having this problem but no solutions so far. anyone here experiencing it? Clean ios btw straight from the apple factory.