This is what I was hoping too. In fact, I believed that it would and placed my order. However, after seeing this article and doing MORE research, the iMac5k will not act as a second monitor to any other device. There was serious confusion with Apple Sales about the difference between 'supporting' extended displays and offering Target Video Mode. That said, I have cancelled my order.
To be clear, this iMac5k can support having it's own second monitor. It CANNOT as a second monitor.
Very disappointing for this price point and loss of functionality from an existing lineup.
Wow. Ok. Thanks for the clarity on this. I see after doing my own research that you are indeed correct. Bummer. I'm torn now. Get a non retina imac that I can use as a monitor down the road or a retina imac that I cannot. Both seem equal spec wise. Guess I'll wait to see the retina imac in person. Then do some serious thinking.