So, as far as I can tell... BOTH brand new iMacs (21.5" & 27") support Target Display Mode, but only with other, new Thunderbolt equipped Macs. The last generation iMacs (27") supported Target Display Mode from any DisplayPort equipped Macs. The 21.5" did not support any Target Display Mode. Is the above information correct? So far thats what I've come to understand, please correct me if I'm missing something!