This appears to be a hardware bug. See http://www.thinkwiki.org/wiki/Problem_with_video_related_system_lockup and http://mailman.linux-thinkpad.org/pipermail/linux-thinkpad/2006-March/032672.html
Adding
Option "BusType" "PCI" Option "DmaMode" "None"
to the "Device" section in xorg.conf fixed the issue for me.
This appears to be a hardware bug. See http:// www.thinkwiki. org/wiki/ Problem_ with_video_ related_ system_ lockup and http:// mailman. linux-thinkpad. org/pipermail/ linux-thinkpad/ 2006-March/ 032672. html
Adding
Option "BusType" "PCI"
Option "DmaMode" "None"
to the "Device" section in xorg.conf fixed the issue for me.