Problem in wp-includes/user.php

I just upgraded from 2.9.2 to 3.0RC3. The public site is working, and I can see the dashboard, but if I try to do anything with Users in the dashboard, I get this:
Fatal error: Call to undefined method wpdb::get_blog_prefix() in /home/xxx/public_html/wp-includes/user.php on line 445

In user.php, around line 445, I see:

`
443 // Initialize
444 $id = (int) $blog_id;
445 $blog_prefix = $wpdb->get_blog_prefix($id);
446 $result = array();
This is not a multi-site or MU.

How can I fix this?

EDIT: I can’t create a new post or edit a post, either. When I try, I get this: Fatal error: Call to undefined method wpdb::get_blog_prefix() in /home/xxx/public_html/wp-admin/includes/user.php on line 260