I still think it's a glitch. For the main reason that they've put Folders in there, which is far more system intensive than a simple still image behind the app icons.
Also, the very fact that you can already set a wallpaper for the lock screen means that it's merely an extension of what's already available.
I refuse to believe I'll have a phone that can run games like Street Fighter 4, be given the new 3D dock, run the Folders feature, but not be able to display a simple 480x320 still image behind the app icons.
You're on a dedicated iPhone forum, and the massive OS4 update was announced and released in beta form yesterday. Hmm, wonder what else he would be talking about.......