I had that sort of thing happening to me with the old OS2005 when I went to sleep playing mp3s. The sound service would not properly let go of the device or something and it would even sometimes manifest itself with clicking and snapping noises that would wake me up. The odd time it would lock up like you indicate and would require me to pull the battery. Sometimes it would keep the DSP powered and drain the battery dead if it wasn't plugged into the charger.

I haven't experiened any of that with the new OS2006 though. However, I generally keep the 770 plugged in over night now and rarely put the cover on while charging. So that might have more to do with it.