If so, WHY AREN'T YOU USING DISPLAYFUSION?!
http://www.binaryfortress.com/displayfusion/Insanely. Freaking. Useful. 'nuff said.
It uses the tiled wallpaper trick, like I've always used, but it takes a lot of the manual labor out of it. It allows you to independently change the wallpaper on each display, saving you a trip into Photoshop. It also has several more options for how to resize wallpaper (i.e. stretch while maintaining aspect ratio, with and without cropping, etc) so it could very well be useful with single display systems as well. Additionally, it has the ability to search for and use pictures from Flickr. I haven't used that feature yet, but it could prove interesting when I grow tired of my current prairie dog/dragon combination.
It's free, and for $10 you can update to the Pro version, which allows you to have it randomly select wallpaper, gives you more control over image cropping, etc. I think the free version is worth $10, myself, but what can you do.