Forwarding this bug from Ubuntu reporter Alwin Garside:
http://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-ati/+bug/555298

[Problem]
User switching causes GPU lockup and black screen

[Original Description]
When switching users in Lucid, often X will freeze up and just give me a black screen with blinking cursor. When I try to go to the current session with Ctrl+Alt+F7,8,9,etc. I either get black, distorted or a non-responsive unlock screen.

DistroRelease: Ubuntu 10.04
Package: xorg 1:7.5+3ubuntu1
ProcVersionSignature: Ubuntu 2.6.32-19.28-generic 2.6.32.10+drm33.1
Uname: Linux 2.6.32-19-generic i686
Architecture: i386
CurrentDmesg:
 [   13.377233] usb 3-2: usbfs: interface 0 claimed by usblp while 'usb' sets config #1
 [   19.436009] eth1: no IPv6 routers present
 [   45.528055] end_request: I/O error, dev fd0, sector 0
 [   45.612036] end_request: I/O error, dev fd0, sector 0
Date: Sun Apr  4 21:08:10 2010
DkmsStatus: Error: [Errno 2] Bestand of map bestaat niet
InstallationMedia: Ubuntu 10.04 "Lucid Lynx" - Beta i386 (20100318)
MachineType: HP Pavilion 06 DF168A-ABH T170.
ProcCmdLine: BOOT_IMAGE=/boot/vmlinuz-2.6.32-19-generic root=UUID=1ed36f90-6dd8-4916-8da8-acd31abccb32 ro quiet splash
ProcEnviron:
 LANG=nl_NL.UTF-8
 SourcePackage: xorg
dmi.bios.date: 07/16/2003
dmi.bios.vendor: Phoenix Technologies, LTD
dmi.bios.version: 3.23
dmi.board.name: MS-6577
dmi.board.vendor: MICRO-STAR INTERNATIONAL CO., LTD
dmi.board.version: 030
dmi.chassis.type: 3
dmi.modalias: dmi:bvnPhoenixTechnologies,LTD:bvr3.23:bd07/16/2003:svnHPPavilion06:pnDF168A-ABHT170.:pvr03Y0211RE101XENO4:rvnMICRO-STARINTERNATIONALCO.,LTD:rnMS-6577:rvr030:cvn:ct3:cvr:
dmi.product.name: DF168A-ABH T170.
dmi.product.version: 03Y0211RE101XENO4
dmi.sys.vendor: HP Pavilion 06
glxinfo: Error: [Errno 2] Bestand of map bestaat niet
system:
  codename:           lucid
 architecture:       i686
 kernel:             2.6.32-19-generic

[lspci]
00:00.0 Host bridge [0600]: Intel Corporation 82845G/GL[Brookdale-G]/GE/PE DRAM Controller/Host-Hub Interface [8086:2560] (rev 03)
	Subsystem: Micro-Star International Co., Ltd. Device [1462:5770]
01:00.0 VGA compatible controller [0300]: ATI Technologies Inc Radeon RV250 If [Radeon 9000] [1002:4966] (rev 01)
	Subsystem: ATI Technologies Inc Device [1002:2002]