Forwarding this bug from Ubuntu reporter Dylan Taylor:
http://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-ati/+bug/554783

[Problem]
Fan on card stopped during usage, leading to system overheat and lockups on reboot.

Is this a hardware error or software?  How do we tell the difference?  

[Original Description]
While I was using my computer, doing perfectly normal things, such as reading news articles in Firefox, my graphics card's fan suddenly completely stopped, causing the GPU to overheat. At first I thought that the fan was slowing down because there wasn't anything particularly GPU intensive going on, and system load was below normal, however, like 10 minutes after, the screen turned completely black, and the computer locked up. This almost certainly because the GPU overheated. The first thing I did was tried Alt-SysRq-K to reboot the X server. That did nothing, so I tried Alt-SysRq-REISUB, and the system did NOT reboot. I'm assuming there may have been a kernel panic. Therefore, I held in the power button on my computer until it turned off. Shortly after that, I hit the power button again to turn the computer back on, but immediately after pressing the power button, the system fans started spinning at maximum speed for a split second and the system immediately shut off. No BIOS POST information was displayed on the screen or anything. So, I assumed the system shut off because it overheated and it was trying to protect the hardware. I let it sit for a couple of minutes, and then I managed to get it to turn on. Something worth noting here is that the GPU fan worked perfectly fine after a full system reboot, making me believe this is some sort of driver problem. The first time I tried to boot it, Ubuntu froze before being able to load the splash screen, so I assumed it may either be because the GPU was still overheated and not working properly, or some file system corruption. I tried Alt-SysRq-B to reboot it, but surprisingly that did nothing, so I turned it off with the power button again. I then booted into recovery mode, and thankfully, I managed to get to the recovery options. I then proceeded to run the options "dpkg, clean, and resume", but when it finished loading services, I was in a text mode login instead of GDM, so I rebooted using Alt-SysRq-REISUB. I then booted Ubuntu with the normal boot method, and after running a quick fsck, I was able to login and post this bug report on the same computer.

DistroRelease: Ubuntu 10.04
Package: xserver-xorg-video-ati 1:6.12.192-2ubuntu2
ProcVersionSignature: Ubuntu 2.6.32-19.28-generic 2.6.32.10+drm33.1
Uname: Linux 2.6.32-19-generic x86_64
Architecture: amd64
Date: Sat Apr  3 15:40:04 2010
DkmsStatus:
 virtualbox-ose, 3.1.4, 2.6.32-17-generic, x86_64: installed 
 virtualbox-ose, 3.1.4, 2.6.32-16-generic, x86_64: installed 
 virtualbox-ose, 3.1.4, 2.6.32-19-generic, x86_64: installed
InstallationMedia: Ubuntu 9.10 "Karmic Koala" - Release amd64 (20091027)
MachineType: Shuttle Inc SG33
ProcCmdLine: BOOT_IMAGE=/boot/vmlinuz-2.6.32-19-generic root=UUID=f75382cd-2436-4b42-96ef-0c1f545f6fa5 ro quiet splash
ProcEnviron:
 PATH=(custom, user)
 LANG=en_US.utf8SourcePackage: xserver-xorg-video-ati
dmi.bios.date: 04/25/2008
dmi.bios.vendor: Phoenix Technologies, LTD
dmi.bios.version: 6.00 PG
dmi.board.name: FG33
dmi.board.vendor: Shuttle Inc
dmi.board.version: V10
dmi.chassis.type: 3
dmi.chassis.vendor: Shuttle Inc
dmi.chassis.version: G5
dmi.modalias: dmi:bvnPhoenixTechnologies,LTD:bvr6.00PG:bd04/25/2008:svnShuttleInc:pnSG33:pvrV10:rvnShuttleInc:rnFG33:rvrV10:cvnShuttleInc:ct3:cvrG5:
dmi.product.name: SG33
dmi.product.version: V10
dmi.sys.vendor: Shuttle Inc
system: codename:           lucid
 architecture:       x86_64
 kernel:             2.6.32-19-generic