I was playing around with my PowerBook connected to a TV. I had it setup as an extended desktop, and was viewing some photos in iPhoto. I was doing some full screen editing on the TV. Since my TV is a lower resolution than is my PowerBook, I could still see part of the iPhoto window on my PowerBook's screen.
So I dragged that window fully back to my PowerBook screen. This had no effect on the full screen image on the TV. I was then able to go into full screen editing mode on the PowerBook screen as well. I was now looking at two distinct full-screen editors.
Since you can view multiple images at once with iPhoto, there may not be such a big need for this. Yet it may be useful for those that have smaller screen real estate.
[robg adds: I haven't tested this one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061213073217396