It is a problem with power management. I have the exact same problem. It also happens sometimes when I turn the backlight off by holding the power button for 2 seconds and then shut the device off. It won't come back on without a reboot. It also happens to me after i sync. something to do with when the backlight setting changes from exteranl power to battery. Lots of people have this problem. Most likely the only fix will be a rom update from hp.
