I just stumbled upon a wonderful website called Interfacelift.com which allows you to download BlackBerry compatible wallpapers. While the site is not intended for BlackBerry devices you can use the wallpapers intended for the iPod (320×240) and throw them onto your Berry. I know all of use Zen theme users will appreciate the multitude of background options.
This will not work for shuretype devices but will work with the full keyboard 320×240 devices like the BlackBerry 8800. Just click the drop down menu underneath every picture and select the iPod option.