If after upgrading to Wordpress 2.3 you're getting database errors similar to
SELECT cat_ID AS ID, MAX(post_modified) AS last_mod FROM `posts` p LEFT JOIN `post2cat` pc ON..., it might be because you're using the Google Sitemaps Plugin. (That link wasn't the exact problem I was having, but I recognized the SQL statements as being similar.)
Easy fix - just download the new version (zip file). 