I believe there may be a regression in thermal control which was introduced in karmic a few days ago. Looking at the packages that aptitude updated, I believe the problem may have started after the linux-image package was upgraded from 2.6.31-10.32 to 2.6.31-10.34. But of course, we ddi see a lot of fairly intrusive changes in a lot of key packages during the last few days as well.
My Thinkpad X24 shut down a couple of times over the last few days without any warning. It's never done that before. I believe the machine may have shut down on its own to prevent damage from overheating. I should say though that the machine wasn't really doing much (but I think Firefox kept it busy nonetheless with the gazillion of tabs I have open all the time ;-)). The machine felt only mildly warm when touched, the fan had been running constantly but at a fairly low speed (judging from the little noise it was making). I added information from the temp sensor to conky and indeed, temperature was going straight for 92°C. At that point, I throttled CPU frequency and the temperature dropped sharply.
To me, it looks like the fan control is not in line with the temperature. 92°C should have the fan going full blast, but I don't think it was going that hard.
$ cat /proc/acpi/thermal_zone/THM0/trip_points
critical (S5): 95 C
passive: 92 C: tc1=5 tc2=2 tsp=600 devices=CPU0