Just thought I'd publish a few findings on this topic. There are a lot of threads about MBPs running hot.
Background - 1.83 GHz Core Duo 15" MBP. Machine has always run pretty hot, but never shut down due to an overheat. After upgrading this machine with a Hitachi Travelstar 2.5" 500GB 7200 rpm drive and another 1/2 GB of RAM, I tried doing a ime Machine backup to an external drive (500 GB 2.5" USB 5400 rpm drive plugged into a hub). It got about 5-10 minutes into the backup when the computer just shut off. The machine didn't shut off doing normal tasks, but my wife had a couple of shutdowns when she was doing some movie editing (very large files), even prior to swapping out the original 80 GB hard drive. Initially I thought the RAM was defective or the power supply, but I put the original RAM and 80 GB drive back into the computer and was able to reproduce the shut down by copying many GB of files, or running Rember (RAM test SW), and even loading OS X in from a DVD.
I found two very helpful pieces of software: The widget 'iStatPro" and the System pref "Fan Control"
I watched the temperatures during the full Time Machine backup (376 total GB).The CPU temperature would get up to 250 F on transfers of very large files (like movies) or running Rember RAM tests. Anything where sustained CPU % utilization stayed up above 80% or more for minutes at a time. It would just completely shut down.
I used the Widget "Fan Control" to set the Base fan speed level to 2800 rpm, set the lower temp threshold to 110F at that speed, and set the upper threshold temp to 160F where the fan would reach 6000 rpm.
I sat and watched a complete 376 GB Time Machine backup run, and a few times during the process the CPU utilization went to 90+% for several minutes - the same point it shut down before. This time it topped out at a CPU temp of about 230F and squeaked through. Most of the time during the backup the temp is only about 130-135 deg F when CPU utilization is down in the 20% or less range continuously.
Bottom line is that this model has a very low thermal margin. Over the years as files have gotten bigger (i.e.higher resolution movies), that margin was finally exposed. The control of the temperature trip points and lower threashold speeds helped it get through.
What I didn't get to the bottom of though was whether or not the heat sinking of the CPU is as good as it was when it was new. I didn't dig into that, and don't know how that heat sink is designed. There was nothing visibly weird looking just looking at the board with the top keboard cover assy off. There wasn't excessive dust in the machine.