Since Lumia doesn't have a user replacable battery and there are occasions in which the entire OS freezes. Thankfully I haven't seen this in my Lumia 900 but with my previous devices where even the power button doesn't work. Under such circumstances the only way is to unplug the battery and plug it again. Now in case of Lumia 900 if situation like this happen, what would be the solution?
When I was using an HTC One X it behaved the same way. That being said, the power button + volume key(s) didn't work and it eventually power cycled on its own. In theory it should work, but I'm sure as is with any kind of technology there could be a glitch.