“Headers Already Sent” error after upgrade to WP 2.5

After not upgrading for several versions, I made the jump to 2.5. Outside of a couple really old plugins needing some minor rewriting, everything seemed to go okay. Then, I went to publish a post and got:

WordPress database error: [Table ‘ryumaou_netgeek.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/ryumaou/public_html/hoffman/netgeek/wp-includes/wp-db.php:216) in /home/ryumaou/public_html/hoffman/netgeek/wp-includes/pluggable.php on line 689

Just to make sure, I cleaned out the wp-includes directory and reuploaded it, but I’m getting the same error on any kind of save operation. The post does go through and either save or post, in spite of the error. And, everything seems to display okay.
I have tried this on two different machines, after clearing cache, just to make sure. I get the same error on both.

Can you tell me what I’ve missed here?

Thanks!
Jim