I'm having the exact same problem.
I keep the laptop running 24/7 on AC power, and for the first 8 months ran OC'd continuously, both FSB and GPU. Didn't have any problems. Eventually updated the BIOS and installed SP2, no noticeable problems after doing so.
Several months ago I experienced a couple of the random shutdown events. I don't remember whether the fans kicked on first or not. I assumed I was running it too hard, and set everything back to stock speeds. That took care of the problem... for the time being. Then I also got an AirDesk, which means it's sitting on a flat surface at all times instead of on my lap when being used, so that should have provided additional help with keeping cool.
Last night it shut down on me while surfing the web. I noticed the fan kick on full speed, and then down it went. No restart, just off. I fired it back up, then went off to other things, so it was just sitting there idling. About an hour later I heard the fan fire up then stop, so I went over and checked it, and it was off again.
I plan to open up the case and try to clean out any dust. But I'm not sure if that's really the problem. I would think the fans would run for awhile, and if it couldn't maintain temp, THEN it would shut down. Also, there's no way it should be overheating when idle, and somebody here said it never shut down when playing games, which I would think would be the worst case for heat. With the fan kicking on for a couple seconds then shutting down, I wonder if the temp sensor / fan controller is at fault, in which case, cleaning it out may not do any good.
FYI, my power settings are 'always on', hibernation is disabled, and I rarely put it to sleep. It just stays on.