I just want to say that I have the same problem with the phone. After some heavy usage, e.g. taking tens of phots, or using GPS continiously for 10 mintues, sometimes (not always), and after I close the screen manually, the phone now respones to nothing! It behaves like zero power! I cannot restart, cannot do anything.
And when I plugin the power, the phone becomes live again. I can see that there is enough power left. So definately a software issue! Nokia, please fix this!