Check this out....
A custom theme maker for WP7.... Must be at least dev unlocked to use...
Themes for Windows Phone, first release - Windows Phone Hacker
You can use any background image you want (well, within size for your tiles). Or you can make a custom look for your tiles.
Only flaw but,
a MAJOR one, it's a image that goes over the tiles, so live tiles dont work at this point. So your tiles will not update, no voice mail, no email, no missed calls...
Too bad, I wont use it with out live tile support...isn't that the point of WP7 ?
Still cool... I could see some great possablities with this if they find a way around the tiles updating...