Very annoying result of a WordPress upgrade today — all of the Categories on the site got squirreled-up in the database upgrade process. Not sure yet whether I’ll try to restore the (overly long and unwieldy) existing list or take the opportunity to clean house. Either way, it’ll be a pain, so for now I’ve just hidden the Category display for articles and the sidebar. If you’re upgrading to WordPress 2.6, be warned that this could happen. Technology is such a joy.
What version of WP were you upgrading from?
It was 2.0.2, I believe. I’ve also seen the same problem described in detail as the result of an upgrade from WP 2.3. Here’s a good article about it and a follow-on piece with one solution (though only a solution for the database-savvy).