This may be apropos of nothing for a lot of people, but just as a helpful little hint, you can get the theme used by a blog by calling the get_option() method thusly:
get_option('current_theme');
I’m using this on a plugin that changes the colors of certain objects (not CSS directable) by checking the current theme and setting matching colors. Think “custom color YouTube video window.”
MU admins, you could use get_site_option() if you’re planning on doing something global.