iPhone 3G will not turn on. Now what?

My iPhone won't turn on. It was connected to the USB port on my IBM Thinkpad laptop, which was plugged into a power source. I was using it all afternoon, then looked at it and the screen was off (normal). I went to turn it on and nothing. Hold the power button to reset it -- nothing. No response at all. And yet the phone feels warm -- hotter than normal.

It's been unplugged from everything for over 90 minutes now, and it still feels hot, and still won't turn on. Phone calls placed to my iPhone ring and ring, then go to voice mail.

Any suggestions?