more info:

I've never put in an SD card

I get them all the time.... something like every hour or two.

I have no programs installed, I just did a hard reset, haven't installed any more programs, and I'm still getting them regularly.

I have a theroy: I think it has to do with the device going to sleep to save power, and just not being able to wake up afterwards.

Every time it happens, the LEDs are still blinking, so I know it's not totally unresponsive.

Also - It's never happened when I've had it hooked up to the power, as I have it set not to turn off when it's on power.