OS X Lion: Unlocking a screen saver as an administrator

With OS X Lion v10.7.3 and later, an admin user can can unlock another user's screen saver.

Unlocking a non-admin user's screen saver

Press Option-Return, then enter an administrative user's name and password.
 

Unlocking an admin user's screen saver

To enable this feature, first use the following advanced steps:

1. As an admin user, edit the /etc/pam.d/screensaver file using the instructions in this article.
2. Locate and remove (or comment out) the following line:
   
   account    required       pam_group.so no_warn deny group=admin,wheel ruser fail_safe
3. Save the changes.
4. Log out and back in.

To unlock the screen saver, press Option-Return, then enter an administrative user's name and password.