Well, I completely disagree with Matan about the on/off/boot vs. idle case. When your battery gets older, as the one in my N800, you'll find that rebooting completely floors the battery. It drops down 3 bars and stays there.

I absolutely prefers to leave my N800 on at all times. I had the occasional runaway-during-the-night problem in the past, but that all disappeared after I got rid of the maemo crawler application. It has never happened again. A couple of months back I reported that my N800 had been dead a couple of times, it turned out that this was caused by a mechanical problem which momentarily disconnected the battery and had nothing to do with the old problem.

Conclusion: Absolutely no software runaway problem ever again for me. And even if there were, the cost of a powerdown/reboot cycle every morning is outrageous compared to a hypothetical runaway case every few months.