I too have had this issue occur on one of my MBPs (early 2009 15" 2.4 A1286).
I would launch steam in windows bootcamp environment and temps would go to mid/upper 60s. Playing simple game such as AOE III (circa 2006 game) and after 10 minutes of gaming the screen goes black. Game sound is still on with the screen black. Requires hard reboot. I installed Lubbos Fan Control to occaisionally monitor temps while in game (using windowed game mode or via alt+tab to see the temps). Temps would be at mid to upper 80s. So I reasoned that it was a thermal efficiency issue.
The laptop is about 4 years old so I bought replacement fans incase they were struggling to spin/cool efficiently. I also did an SMC reset since some of the thermal charateristics of the Mac are controlled by SMC. Ordered new thermal grease too.
Since installing the new fans, SMC reset, applying the thermal grease to the GPUs and to the CPU... temps in OS X at idle are low to mid 40s (30s at startup in OS X). Temps in windows bootcamp environment are at about 62c at startup but settle to mid/high 40s at after a few minutes at idle (startup in windows has alot of services restarting, etc which accounts for the 60s temps at startup). SImple IE browsing is at low 50s (currently at 52c as I type this).
Now when I launch Steam, the temps while in windows climb to 60s. When in AOE III, temps can still climb to mid/uper 70s but never over 80c. I haven't had the black screen appear again, but I'll continue to test and monitor to be certain it doesn't return. One thing is certain, the temps are alot better now.
Macs running windows via bootcamp just simply run hotter than in OS X when doing similar things (Just launching steam in wondows, without running a game causes a 10c increase), but you can do a few things as noted above to try to keep the temps down.
Hope that helps.
Ben
