@Miek
True, yet that is what triggered my (still unanswered) question...
On Windows 7 I've ran into some ickyness as well from time to time (self-inflicted, the system will go haywire if you run PowerShell as admin while calling some specific methods on wmi or com objects) but so far always managed to get out of that through use of control-alt-delete.
It raises an event which is then picked up by the OS core which then allows you to perform a few tasks (lock computer, logout, restart, start task manager). However, one would expect that Win8 would also have such a key event.
But this story seems to indicate that there are no (hardware) keys which one can use to reset the OS. Which I think could be a very big problem if the device becomes totally unresponsive.
Sure; worst case scenario you can always simply take out the batteries and then power it back on, but for something which is marketed as user friendly that seems a bit drastic to me.