Erasmus Probably has the spec pretty close.
If this will help your research, it is the same technology Apple used on the Aluminum MacBook Pros' iSight activity indicator - it illuminates green when active, and is virtually invisible when off.
I agree with the earlier poster - it's these touches and finish that make Apple's designs so attractive. I consider many works of art (I believe many of Apple's designs have actually been displayed as such through the years).
(This tech is also used for indicators on the Apple BT Keyboard, Magic Trackpad, Apple BT Headset, and UB MBP's battery level indicators.)