There are some other threads which you should look up, they describe the possibility of more or less 'invisible' alarms, causing this.

What's a bit funny is how we seem to go through phases of different problems.. I had never heard about any automatic boot problem before, and when I get in after the weekend there's suddenly a bunch of threads about it.. almost like problems are scheduled to appear at certain places in the calendar, to keep us occupied

EDIT: The reason it looks as a new problem to me is because several threads mention this to be caused by alarms. Some reboots we've discussed in the past appeared to instead be an out-of-memory problem, somehow related to the dsp.