I'm not a coder, but figured this one out. I've got two WP blogs running the same theme. Both were upgraded this morning from 2.9.2, using a simplescripts upgrade via my host (bluehost.com). One worked fine. The second returns a fatal error, reproduced here:
Fatal error: Call to a member function add_rewrite_tag() on a non-object in /home6/michaev1/public_html/wp-includes/taxonomy.php on line 289
The culprit? All plugins are the same on each site except the site that didn't work was running WP-Ban.
I restored, deactivated the plugin and retried the upgrade. It worked.
The developer stopped updating his plugins a while ago. I suppose WP Ban is down for the count.