Blue Boot

Here's a Plymouth theme I knocked up using the Moreblue Orbit wallpaper (which is in the Debian desktop-base folder), and the Debian Vizta Plymouth theme.
I've added the Moreblue Orbit wallpaper as a background image to the theme, as described here, changed the Debian logo used to the standard logo, and changed the position the logo swarm appears by editing the Plymouth script.
The GDM splash screen is just the Moreblue Orbit wallpaper minus the Debian logo.
In contrast to my dark Plymouth theme, the lack of integration between Plymouth and the GDM greeter is obvious- the screen turns black for several seconds and the Gnome cursor timer appears. Plymouth themes in Fedora go straight from the Plymouth theme to the GDM splash- without a black screen appearing, so I imagine better integration will arrive at some point in Debian.

Change GDM background image in squeeze

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).