If you want to use an iWork Numbers spreadsheet as your desktop wallpaper, just save it as a PDF and then use iWork’s export feature. Note that you’ll get a bit hosed with the very top and bottom border of the spreadsheet (seems iWork just won’t give you control over the full background color) but you can get a result that you’ll probably be happy with.

The basic idea is to first use your screen resolution to calculate a custom “Paper Size”. For example, if you’re on a MacBook Air you’re resolution would be 1280 x 800, and at 72dpi, that would be 17.77 x 11.11 inches, or 45.15 x 28.22 centimeters.

You can add a shape to the background to set a custom size… Just don’t set Mac OS X to swap your wallpaper every 5 seconds or the system will crawl to a halt – at least if you’re working with PDF files…