Right now I have a mid-2011 iMac, and next to it an ageing mid-2009 MacBook Pro. I'm looking to buy a new rMBP when they get updated, and I thought when I do, I'd be able to use the iMac as an external display in Target Display Mode. I saw a YouTube video showing how to do this, but I can't seem to find out if it's possible to do this with the rMBP lid closed (aka "clamshell mode'). Would that be possible? Thanks!