Update 2010-12-06: Ray points out that you can do this with gconf-editor, which is a much better solution.
It’s a handy feature of Gnome that pressing “PrntScrn” or “Alt+PrntScrn” will take a screen capture of the desktop or current window, respectively. However, in typical Gnome fashion, you can’t do anything more advanced. You can’t capture a region, or a window area. You can’t delay the capture. You can’t save the captures with iterating, numbered filenames. However, the equivalent KDE tool can do all of those things, and you can make Gnome use it.