I've not really found out why, but for example just now I unplugged my N800 from its charger and I am unable to switch the unit on. It's just bricked. It does it regularly.
2-3 minutes later, it switches on just fine with no fiddling around. I have before removed battery to no avail. Sometimes it bricks when I switch off when mplayer running (I always close first now).

However, it really should respond to the power button, and it can be downright annoying.

Any ideas?

Nick