This is just a quick post for anyone who got the following error (as I did on a couple sites) when upgrading WordPress to 3.0+
Fatal error: Call to a member function add_rewrite_tag() on a non-object in … /wp-includes/taxonomy.php on line 289
This error is USUALLY the result of upgrading wordpress without first disabling all your plugins. The easiest way to fix this error is just to FTP in to your site and create a sub folder under plugins and move all your plugins in to that sub folder. Once you've done that you should be able to go to your wordpress site without any errors.
After that you can just log in to the admin panel and go to plugins you will see error messages at the top of the screen for each plugin you moved. The error is just that wordpress couldn't find the plugin so it disabled them. Once you do that you can move the plugins back to the plugins directory (out of the sub folder) and reactivate them 1 at a time.
On a side note be sure to upgrade any plugins that have available upgrades before reactivating them.
Hope this helps someone not want to pull their hair out.