BuddyPress.org

Just read over your post again and you say you want to eliminate the entire backend? Off the top of my head, I don’t know if that’s possible.

Best way to do this would be to integrate the P2 theme into your user blogs.

Otherwise, I know there is a WPMU plugin out there to help limit the admin side bar’s available links. If you manually navigate to the page you can still get to them, but it hides the options from view.