Hmm, I never really found out a way to do it.
Obviously the background can be changed by replacing the actual backgraound image. (somewhere in /usr/local/share I think)
Normally I would suggest this being a bad idea, but the only alternative is using gconf to edit the setting, which is not pretty. (Kinda like raw registry editing for windows)
The older gdm was a lot more customizable, just change some XML to change the design. It was very good.
