When I got my iPhone this morning it had an 80% charge. I plugged it in to activate/charge and I noticed it did get a lot hotter than my old 3G would when charging. I'll keep and eye on it the next couple days.
My phone became very hot when plugged-in to an AC outlet via USB adapter! When I realized this I unplugged it and finished charging connected to my iMac's USB port...which resulted in the phone cooling down. It also seems to me that the 4 takes longer to charge than did my 3G.
Just got mine a few hours ago activated.. I have a 3gs and an iphone 4.. the 4 gets really hot.. usually on calls or while playing videos.. the metal around the phone makes it hotter.. Not sure if defect, but Im thinking about keeping the 3gs now with all this problems being discussed on 1st launch date.
Mine gets slightly lukewarm when playing mp3/videos, multitasking etc..
Only time I noticed it getting hot in the back and the edges was towards the end of a 30 minute facetime video chat, but it cooled down pretty quick after that call ended