It's disabled by default!
http://xsellize.com/threads/98088-HOW-TO-Enable-Wallpapers-On-iPhone-3G-Running-iPhone-OS-4.0
"There's not, because there are few other things that go with wallpaper, like shadows, gradients, etc. and iPhone 3G slows down with all these stuff. Try it and you will see it."
http://xsellize.com/threads/98088-HOW-TO-Enable-Wallpapers-On-iPhone-3G-Running-iPhone-OS-4.0
"There's not, because there are few other things that go with wallpaper, like shadows, gradients, etc. and iPhone 3G slows down with all these stuff. Try it and you will see it."