This hasn't happened before. I was just checking out the Market, didn't download anything, was just going to check out the Dolphin browser comments when my screen blanked. The phone rebooted itself!
I was using the vertical touch screen the same way I usually do. (Wasn't touching the power button at all.) Maybe the battery is loose?
This has me quite concerned. No new apps, installed flexilis and goggles over the weekend but the reboot just happened now.