I made a very simple GDM greater to match the theme I use for my desktop:
The options button in the bottom left comes from the Human List greater (Copyright 2004-2006 Canonical Ltd.).
You can easily modify the theme to use your own background. Simply extract the contents, replace the background.png file (and the screenshot.png file if you like), repackage it and you're good to go.
The theme was designed specifically for a 1280x800 screen. I'm not sure how it will perform on other screens. If you find any problems or have any requests, let me know.