I believe what the EVO is doing with Face Detection is once a face has been detected, a white box will surround the face. Then the software analysis the face to ensure it is properly focused and once it has found a suitable focus setting the box surronding the face turns green to give you the go ahead to snap the picture...
This is a common feature in some of the better branded cameras such as Canons and Nikkons...
Here's a video demonstaring Face Detection software...
http://www.youtube.com/watch?v=tj93NSIVkOs