Windows 7 support is not highly promoted because it is preliminary. We have been burned before on pre-releases. For example, we had to redo all of the default wallpaper-based logons for the RC1 release.
XP support is on the future features list, but it's at a low priority given that there's a fully-featured (if retro) program available. Our main goal there is to provide feature parity, so that you can use new-format logons on XP as you can on Vista/7.
Incidentally, Microsoft has gone out of its way to find interesting wallpapers this time. Most have a story behind them, and you can find out more by searching on the title and author of the piece. My personal favourites are the shot of Keoneloa Bay in Hawaii, and Melbourne's City Link toll road, though I also loved the photograph of stuffed Bunnies in the beta.
