In Ubuntu Karmic John Rose reported:

Every so often using Karmic, external monitor screen (mirrored to laptop screen) goes blank while laptop screen displays OK. I think that this may be happening when I move the mouse after a few seconds inactivity on my part. One thought is that the following part of the Screen Section (in xorg.conf) should not be commented out:
 # SubSection "Display"
 # Virtual 2048 2048
 # EndSubSection
I have the feeling that this was commented out when I was using Jaunty due to external screen mode display problem.

ProblemType: Bug
Architecture: i386
Date: Fri Mar 5 09:45:03 2010
DistroRelease: Ubuntu 9.10
MachineType: Dell Inc. Inspiron 1501
Package: xserver-xorg-video-ati 1:6.12.99+git20090929.7968e1fb-0ubuntu1
ProcCmdLine: root=UUID=bb939a76-b18f-4e9b-b20d-203005e06ca8 ro quiet splash acpi=force irqpoll
ProcEnviron:
 PATH=(custom, no user)
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
ProcVersionSignature: Ubuntu 2.6.31-20.57-generic
RelatedPackageVersions:
 xserver-xorg 1:7.4+3ubuntu10
 libgl1-mesa-glx 7.6.0-1ubuntu4
 libdrm2 2.4.14-1ubuntu1
 xserver-xorg-video-intel 2:2.9.0-1ubuntu2.1
 xserver-xorg-video-ati 1:6.12.99+git20090929.7968e1fb-0ubuntu1
SourcePackage: xserver-xorg-video-ati
Uname: Linux 2.6.31-20-generic i686
dmi.bios.date: 12/07/2007
dmi.bios.vendor: Dell Inc.
dmi.bios.version: 2.6.3
dmi.board.name: 0UW744
dmi.board.vendor: Dell Inc.
dmi.chassis.type: 8
dmi.chassis.vendor: Dell Inc.
dmi.chassis.version: Not Specified
dmi.modalias: dmi:bvnDellInc.:bvr2.6.3:bd12/07/2007:svnDellInc.:pnInspiron1501:pvrNotSpecified:rvnDellInc.:rn0UW744:rvr:cvnDellInc.:ct8:cvrNotSpecified:
dmi.product.name: Inspiron 1501
dmi.product.version: Not Specified
dmi.sys.vendor: Dell Inc.
fglrx: Not loaded
system:
 distro: Ubuntu
 architecture: i686kernel: 2.6.31-20-generic

http://launchpadlibrarian.net/40235314/PciDisplay.txt

Launchpad ref: https://bugs.edge.launchpad.net/ubuntu/+source/xserver-xorg-video-ati/+bug/532517

Thanks!