WordPress 3.0 will separately identify the Drop-ins that you add to your installation. WordPress Drop-ins are custom PHP files related to several WordPress core files such as advanced-cache.php or object-cache.php that you can load on WP-Content to override the core file with new functionality.
The image to the left is located on a WordPress Trac ticket, and shows how WordPress 3.0 will display Drop-ins. Memcached is one of the plugins displayed on the image, which overrides WordPress’ own object-cache-php file. Check also this article about replacing WordPress Object Caching.