I have a similar problem after upgrading to 3.0. Page nor admin works. I did a complete new install to test. I imported all my info (pages, posts and database, etc) and it was fine until I went back to the correct theme.
I just tried again on another test site that uses a different theme and the same conditions occur.
Fatal error: Call to a member function add_rewrite_tag() on a non-object in /home/upsta11/public_html/test/wp-includes/taxonomy.php on line 289
I don’t think 3.0 was quite ready…