Rather than create a new bug, I'll say I've been having this same problem for a week or so (IBM Thinkpad T40 running single-boot Feisty, kernel version 2.6.20-16-generic #2 SMP).  It has happened both when I try to bring my laptop out of hibernation and when I've just left it running and stepped away for a bit, but it has yet to happen when there's been any recent (within last 5 minutes) activity.

xorg.conf below.

Thanks for your help with this, Bryce (et al.).
Jessica

# /etc/X11/xorg.conf (xorg X Window System server configuration file)
#
# This file was generated by dexconf, the Debian X Configuration tool, using
# values from the debconf database.
#
# Edit this file with caution, and see the /etc/X11/xorg.conf manual page.
# (Type "man /etc/X11/xorg.conf" at the shell prompt.)
#
# This file is automatically updated on xserver-xorg package upgrades *only*
# if it has not been modified since the last upgrade of the xserver-xorg
# package.
#
# If you have edited this file but would like it to be automatically updated
# again, run the following command:
#   sudo dpkg-reconfigure -phigh xserver-xorg

Section "Files"
	FontPath	"/usr/share/X11/fonts/misc"
	FontPath	"/usr/share/X11/fonts/cyrillic"
	FontPath	"/usr/share/X11/fonts/100dpi/:unscaled"
	FontPath	"/usr/share/X11/fonts/75dpi/:unscaled"
	FontPath	"/usr/share/X11/fonts/Type1"
	FontPath	"/usr/share/X11/fonts/100dpi"
	FontPath	"/usr/share/X11/fonts/75dpi"
	FontPath	"/usr/share/fonts/X11/misc"
	# path to defoma fonts
	FontPath	"/var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType"
EndSection

Section "Module"
	Load	"i2c"
	Load	"bitmap"
	Load	"ddc"
	Load	"dri"
	Load	"extmod"
	Load	"freetype"
	Load	"glx"
	Load	"int10"
	Load	"type1"
	Load	"vbe"
EndSection

Section "InputDevice"
	Identifier	"Generic Keyboard"
	Driver		"kbd"
	Option		"CoreKeyboard"
	Option		"XkbRules"	"xorg"
	Option		"XkbModel"	"pc105"
	Option		"XkbLayout"	"us"
	Option		"XkbOptions"	"lv3:ralt_switch"
EndSection

Section "InputDevice"
	Identifier	"Configured Mouse"
	Driver		"mouse"
	Option		"CorePointer"
	Option		"Device"		"/dev/input/mice"
	Option		"Protocol"		"ExplorerPS/2"
	Option		"ZAxisMapping"		"4 5"
	Option		"Emulate3Buttons"	"true"
EndSection

Section "InputDevice"
	Identifier	"Synaptics Touchpad"
	Driver		"synaptics"
	Option		"SendCoreEvents"	"true"
	Option		"Device"		"/dev/psaux"
	Option		"Protocol"		"auto-dev"
	Option		"HorizScrollDelta"	"0"
        Option          "SHMConfig"             "true"
EndSection

Section "InputDevice"
  Driver        "wacom"
  Identifier    "stylus"
  Option        "Device"        "/dev/wacom"          # Change to 
                                                      # /dev/input/event
                                                      # for USB
  Option        "Type"          "stylus"
  Option        "ForceDevice"   "ISDV4"               # Tablet PC ONLY
EndSection

Section "InputDevice"
  Driver        "wacom"
  Identifier    "eraser"
  Option        "Device"        "/dev/wacom"          # Change to 
                                                      # /dev/input/event
                                                      # for USB
  Option        "Type"          "eraser"
  Option        "ForceDevice"   "ISDV4"               # Tablet PC ONLY
EndSection

Section "InputDevice"
  Driver        "wacom"
  Identifier    "cursor"
  Option        "Device"        "/dev/wacom"          # Change to 
                                                      # /dev/input/event
                                                      # for USB
  Option        "Type"          "cursor"
  Option        "ForceDevice"   "ISDV4"               # Tablet PC ONLY
EndSection

Section "Device"
	Identifier	"ATI Technologies, Inc. Radeon Mobility M7 LW [Radeon Mobility 9000]"
	Driver		"ati"
	BusID		"PCI:1:0:0"
EndSection

Section "Monitor"
	Identifier	"Generic Monitor"
	Option		"DPMS"
EndSection

Section "Screen"
	Identifier	"Default Screen"
	Device		"ATI Technologies, Inc. Radeon Mobility M7 LW [Radeon Mobility 9000]"
	Monitor		"Generic Monitor"
	DefaultDepth	24
	SubSection "Display"
		Depth		1
		Modes		"1024x768"
	EndSubSection
	SubSection "Display"
		Depth		4
		Modes		"1024x768"
	EndSubSection
	SubSection "Display"
		Depth		8
		Modes		"1024x768"
	EndSubSection
	SubSection "Display"
		Depth		15
		Modes		"1024x768"
	EndSubSection
	SubSection "Display"
		Depth		16
		Modes		"1024x768"
	EndSubSection
	SubSection "Display"
		Depth		24
		Modes		"1024x768"
	EndSubSection
EndSection

Section "ServerLayout"
	Identifier	"Default Layout"
	Screen		"Default Screen"
	InputDevice	"Generic Keyboard"
	InputDevice	"Configured Mouse"
	InputDevice     "stylus" "SendCoreEvents"
	InputDevice     "cursor" "SendCoreEvents"
	InputDevice     "eraser" "SendCoreEvents"
	InputDevice	"Synaptics Touchpad"
EndSection

Section "DRI"
	Mode	0666
EndSection