I fixed this once before, but apparently the recent re-installation of the ATI Catalyst drivers un-did the fix. X-Windows will attempt to cycle through the available video modes when you hit Ctrl-Alt-Plus or Ctrl-Alt-Minus. Problem is, it doesn't work right and (at least on my system) forces the dual displays into "clone" mode at 1024x768, with no way to switch back to the correct mode (that I've been able to find) without logging out!
I use Ctrl-Alt-<digit> on the numpad as hotkeys to switch between virtual desktops, so it isn't difficult to accidentally hit one of the mode switch key combinations by mistake.
For anyone else who's ever gotten burned by this ill-conceived "feature," you can add the following to your xorg.conf file to disable it:
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Option "DontZoom" "on"