I have a monitor which was properly switched off by my Debian PC when unused. I attached it to another machine and, this times, it is never switched off.
/etc/X11/xorg.conf, I have:
Section "Monitor" Identifier "Generic Monitor" Option "DPMS"
It is recognized when X11 starts:
(II) Loading extension DPMS ... (II) VESA(0): DPMS capabilities: StandBy Suspend Off; RGB/Color Display ... (**) Option "dpms" (**) VESA(0): DPMS enabled
The operating system is Debian 5 (Lenny). The graphics card is:
00:02.0 VGA compatible controller: Intel Corporation 82G33/G31 Express Integrate d Graphics Controller (rev 02) (prog-if 00 [VGA controller]) Subsystem: Hewlett-Packard Company Device 2a6f Flags: bus master, fast devsel, latency 0, IRQ 5 Memory at fe900000 (32-bit, non-prefetchable) [size=512K] I/O ports at b080 [size=8] Memory at d0000000 (32-bit, prefetchable) [size=256M] Memory at fe800000 (32-bit, non-prefetchable) [size=1M] Capabilities:  Message Signalled Interrupts: Mask- 64bit- Queue=0/0 Enable-Capabilities: [d0] Power Management version 2
X.Org X Server 1.4.2 Release Date: 11 June 2008 X Protocol Version 11, Revision 0 Build Operating System: Linux Debian (xorg-server 2:1.4.2-10.lenny2) Current Operating System: Linux ludwigVII 2.6.26-2-686 #1 SMP Sun Jun 21 04:57:3 8 UTC 2009 i686 Build Date: 08 June 2009 09:12:57AM