As you may know, with WordPress 2.7 you can now upgrade through the admin panel without uploading any files, etc. I recently got an error when upgrading automatically from 2.7 to 2.7.1. After a bit of googling I found this blog post: fatal error upgrading WordPress.

Turns out you need to deactivate the WordPress Automatic Upgrade plugin which is basically no longer required. You may not have this plugin installed so you won’t have this problem.