I recently upgraded from Maverick to Natty and this problem has appeared after the upgrade - it never happened while I was on Maverick.
I'll try to explain this as well as I can.
On Maverick when I plugged/unplugged the external display to my laptop I had to manually enable/disable the external display from the Monitors dialog.
By Monitors dialog I mean this:
But on Natty, when I plug/unplug the monitor this is automatically detected and the display is enabled/disabled, without me needing to go that Monitors dialog and do anything. I think that the OS does some polling of the external display port to see if something is plugged.
My problem is, when I have already plugged an external monitor and am using it, sometimes it becomes disabled by the OS, as if I had went to the Monitors dialog and disabled it manually, only I didn't do anything, I am watching a movie and I'm not near the computer at all. I think that this autodetect feature disables it. It seems that it thinks that I have unplugged the cable, even though I haven't. I think that the VGA cable doesn't make good contact with the pins in the laptop's VGA port so it might be disconnecting for a fraction of the second, but this wouldn't be a problem if there wasn't this autodetect feature.