According to AppleInsider, Mac OS X Mountain Lion will feature a new full screen mode that should make it a much less annoying feature for Mac users who have multiple monitors.
"Apple's solution in Mountain Lion is an incremental band-aid, but does expand the usefulness of Full Screen mode to users who connect to external displays. New in Mountain Lion is the ability to target which screen you want to go Full Screen in. In the screen shots above, Safari was taken full screen on an externally connected HDTV.
This allows notebook users, for example, to connect to a big external display and use it for Full Screen work. Unfortunately, all the other screens are still blanked, but there isn't a simple fix to addressing this in a sensible way. "