I just upgraded to WP 2.3.3. (http://lyrical.7and1.net) from 2.0.something. The upgrade itself went fine. My site looked a little buddy until I remember to reactivate the theme switcher (silly me) but now the switcher doesn't work. Clicking on any of the available themes just reloads the page, same thing.
I tried deactivating other plugins in the event that they were conflicting. I only have a few and that wasn't the issue.
I tried switching the default theme. Doesn't matter.
I haven't touched anything with the themes themselves and I did nothing besides upgrade WP, delete some deactivated plugins (unrelated) and delete the index.php from the /wp folder (my index is on the main subdomain) but that was after I found the issue with the switcher and it still hasn't changed.