Some auth extensions can provide central user IDs which are the same over a wiki farm (e.g. CentralAuth's globaluser.gu_id). Some extensions need a global id when installed on a wiki farm (e.g. OAuth so a tool you authorized on one wiki can edit in your name on another). Currently there is no clean way to communicate these IDs - OAuth for example provides a bunch of hooks like OAuthGetLocalUserFromCentralId and OAuthGetCentralIdFromUserName and CentralAuth implements them. This is not a great solution - every centralid-relying extension providing a set of hooks and every auth extension implementing all of them wouldn't scale well. It would be nice if AuthManager provided a mechanism through which this communitaction can be channeled.
AuthManager::getCentralIdFromUser( $user, $providerId ) and AuthManager::getUserFromCentralId( $id, $providerId ) would be a simple way to provide this.