It seems that the phpBB3 developement team was thinking about adding double activation to phpBB3.
language/en/acp/board.php:
// 'ACC_USER_ADMIN' => 'User + Admin',

IMO this option is very important to use the advantages of ADMIN ACTIVATION and EMAIL ACTIVATION together in one option. So newly registered users first have to validate their email address and then administrators are activating the user accounts.

I've done a MOD to add this feature: http://www.phpbb.com/community/viewtopic.php?f=70&t=585061