Here is the second post on removing the SpaceFun theme from Debian Squeeze. This time it's the GDM login screen. GDM3 has no easy GUI to change the background image, so a bit of editing is again required. (If only there were an easy way to change the GDM theme, then Debians adoption of a whimsical theme wouldn't be such an issue!)
I found the answer on the Debian Forum.
It boils down to this: edit the file /etc/gdm3/greeter.gconf-defaults, uncomment the line # Use a specific background, and change the file name to the name of the file you want to use. (Drop an .svg image in /usr/share/images/desktop-base.)
Update: other image formats work too, for example .png.
Update 2: If the image file is not displayed (resulting in the GDM screen being a rather unpleasant shade of green), check the file permissions: root must have access (read only).