In Mountain Lion, you will be able to select which of your two or more monitors you want to be active when you enter full screen mode.
"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. "
In Lion, when you entered full screen mode, the primary display was the one that was used in full screen mode, blanking out any external monitors. This feature is really annoying for, say, MacBook users who used an external monitor in addition to the MacBook's display. While not a complete solution, it will be a little bit better in Mountain Lion.
Mac OS X Mountain Lion is expected to go on sale next week exclusively in the Mac App Store. Mountain Lion will cost $19.99 for all Mac users with compatible hardware.