It is quite easy to span a background image across multiple monitors in Ubuntu 10.10. To begin, download your favorite multi-monitor background from dualmonitorbackgrounds.com. Now go to System > Preferences > Appearance and click on the Background tab:

Click Add and locate the background image you want to add. You will now see its thumbnail listed with the rest of the background images. Select the image and change Style to Span. Your background should now span across multiple monitors like this: