Comment 3 for bug 445100

This problem is affecting me as well, but I don't get a blank screen, but can only see the background image. The gnome panels disappear, and you can only use ALT-F2 to get a command window to change the settings.

Setting the screen res back to 1024x768 doesn't cure the problem, but setting the screens to mirrored and then back top this res does get the external monitor working again, and this is 100% reproducible.

I'm using the x86_64 bit version, so it would appear to be across the board.

More info
Machine Dell XPS1330
Graphics card:
00:02.0 VGA compatible controller: Intel Corporation Mobile GM965/GL960 Integrated Graphics Controller (rev 0c)
 Subsystem: Dell Device 0209
 Flags: bus master, fast devsel, latency 0, IRQ 30
 Memory at fea00000 (64-bit, non-prefetchable) [size=1M]
 Memory at e0000000 (64-bit, prefetchable) [size=256M]
 I/O ports at eff8 [size=8]

I have attached the .xsessions-error file which shows a compiz error.