As reported on Google OS, YouTube now shows a theater view option for some videos:
www.youtube.com/watch?v=zJOS0sV2a24

It's not a full-screen mode (which is the other button below the video), it's an actual theater display, with red curtains and all... odd at first sight.

googlesystem.blogspot.com/2008 ...

hmmm strange.
I can sort of see the point of the "lights off" button, but why add curtains?