I've had this issue since upgrading from a fully working Lenny to Squeeze (via numerous hiccups which I am gradually ironing out...) Now using kernel 2.6.32-5-686 on an Acer Extensa 5510 laptop.
After opening the lid, I would get a frozen system. Sometimes it would respond to CTRL+ALT+F1; on one occasion ps -Al showed two gnome-screensaver processes; killing the first one unwedged GDM. However this was not always the case. Sometimes I could use SysRq to get out of it, sometimes even that wouldn't work.
After reading this thread, installing acpitool and running acpitool -s has now "magically" fixed something; when I open my lid now I briefly see a message about render error but then I get GDM back.
[ 475.015368] render error detected, EIR: 0x00000010
[ 475.015373] page table error
[ 475.015377] PGTBL_ER: 0x00000003
[ 475.015383] [drm:i915_handle_error] *ERROR* EIR stuck: 0x00000010, masking
[ 475.015400] render error detected, EIR: 0x00000010
[ 475.015404] page table error
[ 475.015408] PGTBL_ER: 0x00000003
Not too worried about this for now as the system is working.