Everyone has had a messy desktop once in a while. The desktop is just the most convenient place to store stuff you are working on right now. To solve this, Leopard allows you to create stacks which are collections of files, programs or folders. For example, Safari automatically creates a Downloads stack where it puts downloaded files so they won’t clutter your desktop anymore.
Incidentally, I just read a paper by Thomas W. Malone titled How Do People Organize Their Desks? Implications for the Design of Office Information Systems, which clearly says computer systems should take advantage of the fact that people naturally organize their work in piles (or stacks)