No 'Clamshell' mode on an iMac I'm afraid. Its NOT a huge deal, but when I see something I like, like a video, I can throw it up onto the big screen on the wall above the imac.
I take the brightness down to zero on the imac and sit across the room to view the big screen. it would just be nice if the imac screen was black, as is the other smaller LCD that sits along side it on the desk...
No biggy - Just wondering if it was a setting that I'm no aware of
