This does not occur with UXA, only with SNA:

Steps to reproduce:
1) Start desktop environment. I use compiz-0.8 as window manager.
2) Launch glxgears, keep it running.
3) Issue the following command:
   xset dpms force off; sleep 2; xset dpms force on

Expected results:
Monitor switches off and on, nothing else happens.

Actual results:
Monitor switches off and on, glxgears is frozen.

Interestingly, compiz is still responsible when performing above steps.

Now replace glxgears with xscreensaver which can turn off the monitor too, and the issue gets more severe. I usually have to kill compiz to get a usable X session again.

On a side note: I often get such messages in dmesg:
[drm] Setting output timings on SDVOB failed
What do they mean? What is SDVOB? Are they harmless?

lspci -v:
00:02.0 VGA compatible controller: Intel Corporation 4 Series Chipset Integrated Graphics Controller (rev 03) (prog-if 00 [VGA controller])
        Subsystem: Dell Device 0276
        Flags: bus master, fast devsel, latency 0, IRQ 44
        Memory at fe800000 (64-bit, non-prefetchable) [size=4M]
        Memory at d0000000 (64-bit, prefetchable) [size=256M]
        I/O ports at ec90 [size=8]
        Expansion ROM at <unassigned> [disabled]
        Capabilities: [90] MSI: Enable+ Count=1/1 Maskable- 64bit-
        Capabilities: [d0] Power Management version 2
        Kernel driver in use: i915

xorg.conf:
Section "Device"
    Identifier  "Intel"
    Driver  "intel"

    # Output configuration
    Option "Monitor-VGA1" "Samsung SyncMaster SA450 VGA"
    Option "Monitor-DVI1" "Samsung SyncMaster SA450 DVI"

    # Tweaks
    Option "AccelMethod" "sna"
    Option "PageFlip" "true"
    Option "TripleBuffer" "true"
    Option "TearFree" "true"
    Option "VSync" "true"
EndSection