Dear All,
I've posted about this before, but I can now give a clearer
description of my problem (thanks to Tim Green for a tip that helped
me get there).  Any suggestions as to how to make DPMS work correctly
will be welcome.
I have two computers (say 1 and 2), both with built-in LCD screens,
plus two external monitors (A and B) that plug into VGA ports.  The
results of running xset dpms force off (or going to DPMS off mode via
xscreensaver) are as follows:
Computer 1 + no external monitor: built-in screen goes to DPMS off
mode.
Computer 1 + monitor A: monitor A goes to DPMS off mode, built-in
screen was in off mode anyway, from time computer was started with
monitor A plugged in.
Computer 1 + monitor B: monitor B goes to DPMS off mode, built-in
screen was in off mode anyway, from time computer was started with
monitor B plugged in.
Computer 2 + no external monitor: built-in screen goes to DPMS off
mode.
Computer 2 + monitor A: both built-in screen and monitor A go to DPMS
off mode.
Computer 2 + monitor B: built-in screen goes to DPMS off mode, monitor 
B stays on, complaining that I should "check signal cable".
So it seems all individual components work OK, and it's the
combination of computer 2 and monitor B that's problematic.
In each case, the monitor behaviour when the computer is switched off
altogether is the same as that in response to xset dpms force off.
Thanks very much,
Dan
Some details that might help:
Computer 1 has an Intel Corporation Mobile 915GM/GMS/910GML Express
Graphics Controller (rev 03), and runs Fedora 9 with xorg 1.5.2-6.fc9.
Computer 2 has an Intel Corporation Mobile 945GME Express Integrated
Graphics Controller (rev 03), and runs Gentoo with xorg 7.2.
Monitor A is a Samsung SyncMaster 710N.
Monitor B is an LG Flatron W2234S.
xorg.conf from computer 1
# Xorg configuration created by system-config-display
Section "ServerLayout"
 	Identifier     "single head configuration"
 	Screen      0  "Screen0" 0 0
 	InputDevice    "Mouse0" "CorePointer"
 	InputDevice    "Keyboard0" "CoreKeyboard"
EndSection
Section "Files"
# RgbPath is the location of the RGB database.  Note, this is the name of the 
# file minus the extension (like ".txt" or ".db").  There is normally
# no need to change the default.
# Multiple FontPath entries are allowed (they are concatenated together)
# By default, Red Hat 6.0 and later now use a font server independent of
# the X server to render fonts.
 	FontPath     "unix/:7100"
EndSection
Section "Module"
 	Load  "dbe"
 	Load  "extmod"
 	Load  "fbdevhw"
 	Load  "glx"
 	Load  "record"
 	Load  "freetype"
 	Load  "type1"
 	Load  "dri"
EndSection
Section "InputDevice"
# Specify which keyboard LEDs can be user-controlled (eg, with xset(1))
#	Option	"Xleds"		"1 2 3"
# To disable the XKEYBOARD extension, uncomment XkbDisable.
#	Option	"XkbDisable"
# To customise the XKB settings to suit your keyboard, modify the
# lines below (which are the defaults).  For example, for a non-U.S.
# keyboard, you will probably want to use:
#	Option	"XkbModel"	"pc102"
# If you have a US Microsoft Natural keyboard, you can use:
#	Option	"XkbModel"	"microsoft"
#
# Then to change the language, change the Layout setting.
# For example, a german layout can be obtained with:
#	Option	"XkbLayout"	"de"
# or:
#	Option	"XkbLayout"	"de"
#	Option	"XkbVariant"	"nodeadkeys"
#
# If you'd like to switch the positions of your capslock and
# control keys, use:
#	Option	"XkbOptions"	"ctrl:swapcaps"
# Or if you just want both to be control, use:
#	Option	"XkbOptions"	"ctrl:nocaps"
#
 	Identifier  "Keyboard0"
 	Driver      "kbd"
 	Option	    "XkbModel" "pc105"
 	Option	    "XkbLayout" "gb"
EndSection
Section "InputDevice"
 	Identifier  "Mouse0"
 	Driver      "mouse"
 	Option	    "Protocol" "IMPS/2"
 	Option	    "Device" "/dev/input/mice"
 	Option	    "ZAxisMapping" "4 5"
 	Option	    "Emulate3Buttons" "yes"
EndSection
Section "Monitor"
 	Identifier   "Monitor0"
 	VendorName   "Monitor Vendor"
 	ModelName    "LCD Panel 1024x768"
 	HorizSync    31.5 - 48.5
 	VertRefresh  40.0 - 70.0
 	Option	    "dpms"
EndSection
Section "Device"
#	VendorName  "Videocard vendor"
#	BoardName   "VESA driver (generic)"
 	Identifier  "Videocard0"
 	Driver      "intel"
 	BoardName   "Unknown video card"
EndSection
Section "Screen"
 	Identifier "Screen0"
 	Device     "Videocard0"
 	Monitor    "Monitor0"
 	DefaultDepth     24
 	SubSection "Display"
 		Viewport   0 0
 		Depth     16
 		Modes    "800x600" "640x480"
 	EndSubSection
 	SubSection "Display"
 		Viewport   0 0
 		Depth     24
 		Modes    "1024x768" "800x600" "640x480"
 	EndSubSection
EndSection
Section "DRI"
 	Group        0
 	Mode         0666
EndSection
xorg.conf from computer 2
# Xorg configuration created by system-config-display
Section "ServerFlags"
 	Option "DontZap" "yes"
 	Option "DontVTSwitch" "no"
EndSection
Section "ServerLayout"
 	Identifier     "Default Layout"
 	Screen      0  "Screen0" 0 0
 	InputDevice    "Mouse0" "CorePointer"
 	InputDevice    "Synaptics Mouse" "AlwaysCore"
 	InputDevice    "Keyboard0" "CoreKeyboard"
EndSection
Section "InputDevice"
 	Identifier  "Keyboard0"
 	Driver      "kbd"
 	Option	    "XkbModel" "pc105"
         Option      "XkbLayout" "gb,us(euro)"
         Option      "XkbOptions" "grp:alt_shift_toggle"
EndSection
Section "InputDevice"
 	Identifier "Synaptics Mouse"
         Driver     "synaptics"
         Option     "Device" "/dev/psaux"
         Option     "Protocol" "auto-dev"
 	Option	   "LeftEdge"  "1700"
   	Option	"RightEdge"     "5300"
   	Option	"TopEdge"       "1700"
   	Option	"BottomEdge"    "4200"
   	Option	"FingerLow"	"25"
   	Option	"FingerHigh"	"30"
   	Option	"MaxTapTime"	"180"
   	Option	"MaxTapMove"	"220"
   	Option	"VertScrollDelta" "100"
   	Option	"MinSpeed"	"0.09"
   	Option	"MaxSpeed"	"0.18"
   	Option	"AccelFactor"	"0.0015"
   	Option	"SHMConfig"	"on"
EndSection
Section "InputDevice"
 	Identifier  "Mouse0"
 	Driver      "mouse"
 	Option	    "Protocol" "IMPS/2"
 	Option	    "Device" "/dev/input/mice"
 	Option	    "ZAxisMapping" "4 5"
 	Option	    "Emulate3Buttons" "no"
EndSection
Section "Monitor"
 	Identifier  "Monitor0"
 	Modeline  "1024x600" 48.96 1024 1064 1168 1312 600 601 604 622 -HSync +VSync
#	Option	"Above"	"Monitor1"
 	Modeline "1280x768"  80.14  1280 1344 1480 1680  768 769 772 795  -HSync +Vsync
 	Modeline "1280x768"  102.98  1280 1360 1496 1712  768 769 772 802  -HSync +Vsync
 	Modeline "1280x1024"  138.54  1280 1368 1504 1728  1024 1025 1028 1069  -HSync +Vsync
 	Modeline "1280x1024"  108.88  1280 1360 1496 1712  1024 1025 1028 1060  -HSync +Vsync
 	Option	    "dpms"
EndSection
Section "Device"
 	Identifier  "Videocard0"
 	Driver      "intel"
#	Option      "monitor-LVDS" "Monitor0"
#	Option	    "monitor-VGA" "Monitor1"
 	Option	    "Clone" "true"
 	Option	"MonitorLayout"	"LVDS,VGA"
 	BusID	"PCI:0:2:0"
#	Screen	0
EndSection
Section "Screen"
 	Identifier "Screen0"
 	Device     "Videocard0"
 	Monitor	    "Monitor0"
 	DefaultDepth     24
 	SubSection "Display"
 		Viewport   0 0
 		Depth     24
 		Modes   "1280x1024" "1024x600" "800x600" "640x480"
 		Virtual	1280 1024
 	EndSubSection
EndSection
Section "DRI"
 	Group        0
 	Mode         0666
EndSection