Had the laptop for about a year, when in Windows it never happens, but seems to happen frequently in Xubuntu 11.04. I've looked almost everywhere for increasing the fan speeds, because it never kicks into high gear, just a steady pace for idle use. lm-sensors doesn't show fan speeds, just the temps of the CPU.
As far as http://ubuntuforums.org/showthread.php?t=1740140 thread goes, I've already got the latest kernel.
Fan is clear of dust, and it runs at an average temperature until the GPU is needed
$ sensors acpitz-virtual-0 Adapter: Virtual device temp1: +85.0°C (crit = +200.0°C) k8temp-pci-00c3 Adapter: PCI adapter Core0 Temp: +76.0°C Core1 Temp: -49.0°C
Shortly after this it shuts off from overheating. When I can get it to cool off I'll give what I can for an idle temp.
I can't tell if it's a fan speed issue, or a special fan for the GPU that isn't supported.