Announcing the all-singing, all-dancing user rights email notification extension. It slices, it dices; it emails registered users upon a change in their group memberships. Yes folks, for users of MediaWiki 1.5.9 and above* this is *the* extension to download. More information minus the marketing spiel at http://meta.wikimedia.org/wiki/User_Rights_Email_Notification.
* Requires hooks present in 1.5.9 or later. I know, 1.5.9 isn't out at the moment. The hook is simple enough to add, it's a one-line change which I'll document somewhere.
Rob Church