I don't know anything about your particular problem but I had a few cases of the screen locking and unlocking randomly without me touching it. I eventually traced it down to moisture condensing inside the phone. This was on the N900 not N9 so your problem may be completely unrelated but if it started happening after e.g. using the phone in the bath then you might have a chance to fix it by drying it out properly.

Either that or your battery is on its last leg. Have you noticed a drop in the battery life?