If you're using Wordpress 3 on a server with APC and PHP 5.3, you might run into this error:
PHP Fatal error: Call to undefined function wp_dashboard_setup() in <DocumentRoot>/wp-admin/index.php
To resolve this: disable apc.include_once_override in your APC settings:
apc.include_once_override = 0
The same setting also breaks Drupal 7 on my servers with PHP 5.3, so it might be a good idea to completely disable it everwhere for now. :)