Forwarding this bug from Ubuntu reporter djchandler:
http://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-ati/+bug/557829

[Problem]
Laptop overheads with the -ati driver, idling at 70°C and shutting down with moderate load.  Overheating does not occur with -fglrx driver.

[Original Description]
Lucid Lynx 10.04 AMD64 Beta 1

xserver-xorg-video-radeon 1:6.12.192-2ubuntu2
xserver-xorg-video-ati 1:6.12.192-2ubuntu2

AMD AthlonTM X2 Dual-Core QL-65 @ 2.1GHz (running stock speed, Cool 'n' Quiet working, cpu frequency regulation in kernel is working, idles @ 1.05GHz but temperature still goes up)
AMD M780V Chipset
ATI Radeon 3100 IGP (RV610)
720P video native support (I suppose that means hardware implemented)

Laptop (Toshiba L305D-S5959) overheats when running open source driver. Idles at around 70°C at room temperature of ~22°C, have BIOS set to shutdown at 95°C to avoid hardware damage. Computer will shut itself off due to temperature under moderate load such as viewing flash video in Firefox on hulu.com. This has been repeated at least 3 times.

Running  the  proprietary fglrx driver 2:8.721-0ubuntu7 (with no other software changes), the computer idles  @ 45°C (20°C cooler) at room temperature of ~22°C. Watching video on hulu.com does not cause overheating issues.

This is something that could damage computers. If it's under warranty but Ubuntu is found on the hard drive the manufacturer will probably void the warranty. I certainly don't want Ubuntu to get that kind of reputation.

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
NonfreeKernelModules: fglrx
Architecture: amd64
Date: Wed Apr  7 23:42:01 2010
DkmsStatus: fglrx, 8.721, 2.6.32-19-generic, x86_64: installed
InstallationMedia: Ubuntu 10.04 "Lucid Lynx" - Beta amd64 (20100318)
MachineType: TOSHIBA Satellite L305D
ProcCmdLine: BOOT_IMAGE=/boot/vmlinuz-2.6.32-19-generic root=UUID=12291b72-5e88-486e-a7e6-60a57bdeefed ro splash quiet vga=786 quiet splash
ProcEnviron:
 LANG=en_US.utf8SourcePackage: xserver-xorg-video-ati
dmi.bios.date: 09/01/2009
dmi.bios.vendor: Insyde Corp.
dmi.bios.version: 1.80
dmi.board.asset.tag: No Asset Tag
dmi.board.name: Portable PC
dmi.board.vendor: TOSHIBA
dmi.board.version: Base Board Version
dmi.chassis.asset.tag: No Asset Tag
dmi.chassis.type: 10
dmi.chassis.vendor: AMD
dmi.chassis.version: None
dmi.modalias: dmi:bvnInsydeCorp.:bvr1.80:bd09/01/2009:svnTOSHIBA:pnSatelliteL305D:pvrPSLC8U-06P01W:rvnTOSHIBA:rnPortablePC:rvrBaseBoardVersion:cvnAMD:ct10:cvrNone:
dmi.product.name: Satellite L305D
dmi.product.version: PSLC8U-06P01W
dmi.sys.vendor: TOSHIBA
glxinfo: Error: [Errno 2] No such file or directory
system: codename:           lucid
 architecture:       x86_64
 kernel:             2.6.32-19-generic

[lspci]
00:00.0 Host bridge [0600]: Advanced Micro Devices [AMD] RS780 Host Bridge [1022:9600]
 Subsystem: Toshiba America Info Systems Device [1179:ff6a]
01:05.0 VGA compatible controller [0300]: ATI Technologies Inc RS780MC [Radeon HD 3100 Graphics] [1002:9613]
 Subsystem: Toshiba America Info Systems Device [1179:ff6a]