Many of us are Vista users with more than one screen, and we have been complaining for years about XPs lack of being able to set one big picture to span across both monitors. It has been a pain, until now. It is not a very well advertised feature of Vista, but your dreams have been answered.
* Right click on the background and select Personalization.
* Click on Desktop Background
* Select a background image that is at least as wide as the combined resolution of both of your monitors (or scale the image up so it is big enough, else you will get tiling.. it takes some tweaking
* Select the Tile picture positioning option as shown below. This is the only option that will display your background image across multiple monitors.