I have a developer Pre 2 with 2.1 on AT&T. Although I have a few patches installed, I noticed this behavior before I even put it in dev mode, I ran stock for a few days to baseline the phone...
I've noticed it using Feeder, exiting the phone app and exiting the email app. All total maybe 6 times in the last 2 weeks. The phone appears to go through a Luna restart, I'm assuming it isn't a full reset because it takes less than a minute to get the screen back.
I don't keep lots of cards open, and I ran through the diagnostic tests.
Anyone else seeing this sort of behavior?