I've had two reboots during the +two weeks I've had my N800. Both times it happened while it was sitting quietly (with the screen off) on the desk. But I believe the media player was active both times, not actually playing anything. I too suspect a memory full problem, but I have tried later to run a simple monitoring setup ('free'+'sleep' loop in the shell), with no memory leaks showing at all. Clearly I've been unable to set up the right conditions, and it happens so rarely so it's difficult to get to see it.