Fatal Error after auto update to 3.1

I’m running several WP sites and updated all to the 3.1 version successfully — except on one site it led to this:
Fatal error: Call to undefined function wp() in /home/brillian/public_html/wp/wp-blog-header.php on line 14

Google doesn’t bring me anyone else who has had this problem. Any suggestions? (I’ve had the site rolled back to previous state, so the trouble isn’t currently visible; but I would like to keep the site upgraded to the current version of WP)