It's rebooting too frequently for the hold to register.
(Well, that's what I think is happening anyway. I've tried various things but nothing really seems to do anything. The little crank reset button resets it, but it goes right back into the update loop . . . it doesn't stay on long enough for me to look at its files via the USB cable anymore (oops) . . . and I have tried holding the wake and menu buttons together for at least a minute to no avail)