The strange part about it is that when I reboot, I get no error message, just like I shut it down normally myself. If it was overheating or hardware failure, wouldn't there be an error or "did not shut down properly" message? It has happened 3 out of my last 4 play sessions, usually after anywhere from 30 minutes to an hour of play. I don't think it's overheating, because I have monitored my temps after a similar amount of play time and they don't exceed 75 C. I'm lost...

PC is a laptop Toshiba Qosmio x505

Operating System: Windows 7 Home Premium 64-bit (6.1, Build 7601) Service Pack 1
Processor: Intel Core i5 CPU M 450 @ 2.40GHz (4 CPUs), ~2.4GHz
Memory: 4096MB RAM
Available OS Memory: 4020MB RAM
DirectX Version: DirectX 11
GPU: NVIDIA GeForce GTS 360M

Could be hardware failure rather than overheating. When the PSU on my desktop went on the fritz, this was pretty much exactly what would happen: be playing a game, PC would shut down and then immediately reboot. Although in my case I got the "did not shut down properly" message. In most cases, when there's a hard shutdown going on, signs point to either overheating or hardware failure rather than software.

Is there anything in your Event Viewer relating to these shut downs? Even if you're not getting the "did not shut down properly" screen, there may still be a Kernel Power event in the Event Viewer.

Also, which temps were you monitoring? GPU and CPU, or just one of the above? Even if the CPU/GPU has acceptable temps, the other could still be overheating. And since you are on a laptop, have you made sure that it's plugged in, the battery's in working order, and the adapter is functioning properly? It's possible Skyrim is demanding more power from your system than the adapter's able to cope with.