I am an 8Gb 3G user and after the upgrade to iTunes 9 and 3.1 my phone has displayed these same symptoms. I have reset it three times today by holding down the 'Home' and 'Power' buttons much like most of you. This is the first time I have experienced an issue with an upgrade.
The upgrade took close to an hour initially, and upon completion I could not open the iPod app. It would start to open with a white screen then crash back to the home screen in a few seconds. I power reset two times, but that did nothing to correct it. I reconnected the phone to 'SYNC' it. It went through the entire backup again (40 - 45 minutes!), but after that all appeared normal (iPod app worked). Today at work I soon discovered THIS nasty issue when attempting to look at my phone it would not respond. It does not respond when you connect it to a pc via the USB cable either. I called my iPhone from my desk phone and I can hear the ring in my receiver , but the iPhone never rings or lights up. RESET .....again......and again.
One more thing, it may just be me never noticing before, but does your phone appear to stay in a "Searching" state much longer than normal upon reset?
I hope Apple addresses this quickly.