WARNING: at /build/buildd/linux-2.6.38/drivers/gpu/drm/radeon/radeon_fence.c:248 radeon_fence_wait 0x36f/0x3e0 radeon

I had thunderbird, firefox and pidgin running and was trying to switch from firefox to pidgin using the unity bar. Pidgin had its main windows and one conversation window open.
I clicked on the pidgin icon, everything freezed and after about 10s the screen blanked out shortly and presented me the normal screen (unity with firefox running) again, but it was also freezed with the mouse pointer hanging at another position.
Shortly afterwards the screen again blanked out.
This happend 3 times, until I rebooted the laptop.
Apport offered me 3 times the same kernel oops report, so maybe the kernel was hanging in an endless loop when trying to resolve one kernel oops and producing another one instead?

ProblemType: KernelOops
DistroRelease: Ubuntu 11.04
Package: linux-image-2.6.38-7-generic 2.6.38-7.39
Regression: Yes
Reproducible: No
ProcVersionSignature: Ubuntu 2.6.38-7.39-generic 2.6.38
Uname: Linux 2.6.38-7-generic x86_64
AlsaVersion: Advanced Linux Sound Architecture Driver Version 1.0.23.
Annotation: Your system might become unstable now and might need to be restarted.
Architecture: amd64
ArecordDevices:
 **** List of CAPTURE Hardware Devices ****
 card 0: Intel [HDA Intel], device 0: ALC272 Analog [ALC272 Analog]
   Subdevices: 1/1
   Subdevice #0: subdevice #0
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: jan 1671 F.... pulseaudio
CRDA: Error: [Errno 2] Datei oder Verzeichnis nicht gefunden
Card0.Amixer.info:
 Card hw:0 'Intel'/'HDA Intel at 0xf2500000 irq 49'
   Mixer name : 'Realtek ALC272'
   Components : 'HDA:10ec0272,1179ff82,00100001'
   Controls : 19
   Simple ctrls : 11
Card1.Amixer.info:
 Card hw:1 'HDMI'/'HDA ATI HDMI at 0xf2110000 irq 50'
   Mixer name : 'ATI R6xx HDMI'
   Components : 'HDA:1002aa01,00aa0100,00100100'
   Controls : 4
   Simple ctrls : 1
Card1.Amixer.values:
 Simple mixer control 'IEC958',0
   Capabilities: pswitch pswitch-joined penum
   Playback channels: Mono
   Mono: Playback [on]
Date: Wed Mar 30 09:14:52 2011
Failure: oops
Frequency: This has only happened once.
MachineType: TOSHIBA Satellite L500
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-2.6.38-7-generic root=/dev/mapper/sda1_crypt ro quiet splash vt.handoff=7
RelatedPackageVersions:
 linux-restricted-modules-2.6.38-7-generic N/A
 linux-backports-modules-2.6.38-7-generic N/A
 linux-firmware 1.49
SourcePackage: linux
Title: WARNING: at /build/buildd/linux-2.6.38/drivers/gpu/drm/radeon/radeon_fence.c:248 radeon_fence_wait+0x36f/0x3e0 [radeon]()
UpgradeStatus: Upgraded to natty on 2011-03-29 (0 days ago)
dmi.bios.date: 10/28/2009
dmi.bios.vendor: TOSHIBA
dmi.bios.version: V1.80
dmi.board.name: KSWAA
dmi.board.vendor: TOSHIBA
dmi.board.version: 1.00
dmi.chassis.asset.tag: *
dmi.chassis.type: 10
dmi.chassis.vendor: TOSHIBA
dmi.chassis.version: N/A
dmi.modalias: dmi:bvnTOSHIBA:bvrV1.80:bd10/28/2009:svnTOSHIBA:pnSatelliteL500:pvrPSLJ3E-02601JGR:rvnTOSHIBA:rnKSWAA:rvr1.00:cvnTOSHIBA:ct10:cvrN/A:
dmi.product.name: Satellite L500
dmi.product.version: PSLJ3E-02601JGR
dmi.sys.vendor: TOSHIBA