Ron and I whipped this up based on previous code of ours and from the plugin Phillip Studinski (forgot your forum username, sorry!) whipped up from that. :)
(Confused? Is okay. Me too.)
This plugin goes in the mu-plugins folder. It adds a sub-menu to Site Admin, which will list all the themes in use site-wide. At the top of the list will be the most popular theme and a number in brackets of how many are in use. Click on the name, and a drop-down list will appear of all users using that theme, with a link to each user's blog and backend.
We're pretty visual people ourselves, and I like stats and menus in the backend so I know who is using what, thus this plugin was born. (I coded the easy bits; I'll admit it. :D I'm getting really good at adding menus.)
I even made it available in two places:
http://atypicalhomeschool.net/wordpress-plugins/
http://wpmudevorg.wordpress.com/project/Theme-Stats
This is an especially usefull plugin if you've got an issue with a particular theme and you need to know quickly if all users with that theme are having the same issue. My users frequently add uls, lis and even divs willy-nilly to their sidebars and then complain their theme is busted.