In WordPress 3.0, it will have, additional to the Custom Header Image support, also a Custom Background Support. A first snapshot is in the current trunk. To use the functionality, the theme needs in functions.php this line:
After that, there is in your backend under Appearance the menu Custom Background available.
After uploading an image, you have the option to activate “Tile the background” and delete the background image.
Here the result with the new Theme Twenty Ten.