I know, myself, using a multi site WordPress install, plugins dropped into the mu-plugins folder (wp-content/mu-plugins/) are activated automatically. In this case mu representing must use.
However, I am not 100% positive if this is dependent upon the type of install a single site blog install or multi site/network install.
I found an article outlining most of the information about mu and drop in plugins that I know here.
Additionally, if you are looking specifically for how these plugins are loaded I opened up /wp-admin/includes/plugin.php and found get_mu_plugins() on line 279.