Vince Ng created a cool plugin called "Login As" that allowed an admin to be redirected to login to a user's site as the user. This was helpful in seeing issues as they see them as opposed to what i see as an admin.
With 3.0 it just redirects to that user's log in screen. i cant find an updated version that is less than a year old.
The following line of code is the only one that references MU, so Im wondering if this is why it doesnt work.
add_submenu_page('wpmu-admin.php', 'Login As...', 'Login As...', 10, 'loginAs', 'displayLoginAsPage');