WordPress Database Error

I upgraded to 2.3 today, and for the first time, something has gone very wrong. My page loads ok, and I can get into the admin tool and even start writing a post. But after maybe 30 seconds, the section below the tag (which I think would typically show the last saved time) says this:

WordPress database error: [Table ‘jason_wordpress.wp_post2cat’ doesn’t exist]
SELECT cat_ID AS ID, MAX(post_modified) AS last_mod FROM wp_posts p LEFT JOIN wp_post2cat pc ON p.ID = pc.post_id LEFT JOIN wp_categories c ON pc.category_id = c.cat_ID WHERE post_status = ‘publish’ GROUP BY cat_ID

If I click SAVE AND CONTINUE EDITING, I get this error:

WordPress database error: [Table ‘jason_wordpress.wp_post2cat’ doesn’t exist]
SELECT cat_ID AS ID, MAX(post_modified) AS last_mod FROM wp_posts p LEFT JOIN wp_post2cat pc ON p.ID = pc.post_id LEFT JOIN wp_categories c ON pc.category_id = c.cat_ID WHERE post_status = ‘publish’ GROUP BY cat_ID

Warning: Cannot modify header information – headers already sent by (output started at /home/jason/public_html/wp-includes/wp-db.php:160) in /home/jason/public_html/wp-includes/pluggable.php on line 390

I’m fairly technical, but this is over my head – I know very little about MySQL databases. Any suggestions here? I have no idea how to proceed or fix this..