I've got a dual-monitor setup. 1980x1200 is main on the left, 1600x1200 is secondary on the right. Dock disappears on the left monitor.
Left desktop is filled with junk. Stuff I download and then move there from the downloads folder; stuff I copy from external HDD's, laptops or other drives; images that I've exported from Photoshop that I'm going to upload somewhere; you get the idea. Basically that desktop is a big "temp" folder. Every once in a while, when it gets too full, I will do a delete/move of everything on it.
Right desktop is for work stuff. Sometimes individual files, but mostly folders. When I am done with any particular project, it gets moved to the projects folder in my documents directory. I kind of create areas on the desktop (I never have more than 3 or 4 projects at a time) so I can have sort of a visual arrangement of relevant files.
Background is the default Snow Leopard nebula thingy, on both monitors. It's been quite a while since I've really cared about desktop backgrounds. When I first got my own PC, I made various image mosaics, then individual images, then I did the map of the world with realtime updates of the sun/shadow areas... then I just lost interest. Nowadays I pretty much leave it on the default, whether on Windows or on Mac.
As for the monitor split/multitasking... I usually have iTunes on the right-hand monitor. Sometimes that is the only thing running over there. Often I will put Activity Monitor over there as well, although I usually keep it minimized (I like to keep tabs on Firefox's memory usage, no pun intended). I keep my main browser window on the main (left-hand) monitor, but when I have more than 2 browser windows open I will move one over to the 2nd monitor. When I'm working, I will have the browser + Word open on the main monitor, and a couple of folders and various PDFs (in Preview) open on the secondary monitor.
