Both login and usingProfileOf are listed in help as available methods, but when I try either of these I get "You have specified an unknown search method. Use the helper function to display a list of available search methods".
I should mention that the ability to use these methods depends on the user category holding the "View User Profile Details" permission. This wasn't the case in 6.24.1236, where anyone could search for login or useprofileof. I guess it is preferable now with access restricted,