Forwarding this bug from Ubuntu reporter david reeves:
http://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/856423

[Problem]
GPU lockup that might be related to power management and/or screen brightness.

We've received a dozen reports with this same error code in recent weeks.

[Original Description]
not sure what happened
I left the laptop running and the screen goes black . when i try to get the screen back by moving the mouse it wont reappear. the only way i can get it back is to hit the power button then the enter button and shut down the machine

If the dimming of the screen is related, then this crash happens before the screen dims and does not come back. See bug 850749 (and bug 856423, which is a duplicate of this bug)

ProblemType: Crash
DistroRelease: Ubuntu 11.10
Package: xserver-xorg-video-intel 2:2.15.901-1ubuntu2
ProcVersionSignature: Ubuntu 3.0.0-11.18-generic 3.0.4
Uname: Linux 3.0.0-11-generic i686
.tmp.unity.support.test.0:
 
ApportVersion: 1.23-0ubuntu1
Architecture: i386
Chipset: i915gm
CompizPlugins: No value set for `/apps/compiz-1/general/screen0/options/active_plugins'
CompositorRunning: compiz
Date: Thu Sep 22 08:27:50 2011
DistUpgraded: Log time: 2011-09-22 01:18:51.791944
DistroCodename: oneiric
DistroVariant: ubuntu
DuplicateSignature: [i915gm] GPU lockup  EIR: 0x00000010 PGTBL_ER: 0x00000100 render.IPEHR: 0x7d8e0001 Ubuntu 11.10
ExecutablePath: /usr/share/apport/apport-gpu-error-intel.py
ExtraDebuggingInterest: Yes, if not too technical
GpuHangFrequency: Continuously
GpuHangReproducibility: Yes, I can easily reproduce it
GpuHangStarted: Immediately after installing this version of Ubuntu
GraphicsCard:
 Intel Corporation Mobile 915GM/GMS/910GML Express Graphics Controller [8086:2592] (rev 03) (prog-if 00 [VGA controller])
   Subsystem: Hewlett-Packard Company NC6220 [103c:308a]
   Subsystem: Hewlett-Packard Company Device [103c:308a]
InterpreterPath: /usr/bin/python2.7
Lsusb:
 Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
 Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
 Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
 Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
PccardctlIdent:
 Socket 0:
   no product info available
PccardctlStatus:
 Socket 0:
   no card
ProcCmdline: /usr/bin/python /usr/share/apport/apport-gpu-error-intel.py
ProcEnviron:
 
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.0.0-11-generic root=UUID=0a95a656-a945-4a78-b3ea-8ddb454185f8 ro quiet splash vt.handoff=7
RelatedPackageVersions:
 xserver-xorg             1:7.6+7ubuntu7
 libdrm2                  2.4.26-1ubuntu1
 xserver-xorg-video-intel 2:2.15.901-1ubuntu2
SourcePackage: xserver-xorg-video-intel
Title: [i915gm] GPU lockup  EIR: 0x00000010 PGTBL_ER: 0x00000100 render.IPEHR: 0x7d8e0001
UpgradeStatus: Upgraded to oneiric on 2011-09-22 (0 days ago)
UserGroups:
 
dmi.bios.date: 10/24/2005
dmi.bios.vendor: Hewlett-Packard
dmi.bios.version: 68DTU Ver. F.0E
dmi.board.name: 308A
dmi.board.vendor: Hewlett-Packard
dmi.board.version: KBC Version 40.21
dmi.chassis.asset.tag: CNU60603N9
dmi.chassis.type: 10
dmi.chassis.vendor: Hewlett-Packard
dmi.modalias: dmi:bvnHewlett-Packard:bvr68DTUVer.F.0E:bd10/24/2005:svnHewlett-Packard:pn:pvrF.0E:rvnHewlett-Packard:rn308A:rvrKBCVersion40.21:cvnHewlett-Packard:ct10:cvr:
dmi.product.version: F.0E
dmi.sys.vendor: Hewlett-Packard
version.compiz: compiz 1:0.9.5.94+bzr2803-0ubuntu5
version.libdrm2: libdrm2 2.4.26-1ubuntu1
version.libgl1-mesa-dri: libgl1-mesa-dri 7.11-0ubuntu3
version.libgl1-mesa-dri-experimental: libgl1-mesa-dri-experimental N/A
version.libgl1-mesa-glx: libgl1-mesa-glx 7.11-0ubuntu3
version.xserver-xorg: xserver-xorg 1:7.6+7ubuntu7
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.6.0-1ubuntu13
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:6.14.99~git20110811.g93fc084-0ubuntu1
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.15.901-1ubuntu2
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:0.0.16+git20110411+8378443-1