I just completed an upgrade from WP 2.0 to 3.1.1. I used phpMyAdmin to import a database backup into a fresh install (long story). But the upgrade seems to be successful in that the site is now displaying.
All of the parts of the admin section seemed to be working, with one exception: Themes. When I click that link, I get a blank page. A look at the page code shows no code at all!
The site is using the Default theme right now.