Just a PSA, really: The newest iMacs are somewhat limited in their capability to be used as external monitors for your laptop. I’d consider the functionality to be a bonus to begin with, but if you were going to be relying on it, better make other plans. It seems that the DisplayPort on 27″ iMacs (there isn’t a video in on the 21.5″ ones) is a bit picky on what it takes an input from. Using various adapters, iFixit determined that at the moment, the only devices which can use the iMac as a secondary display are those equipped natively with a Mini DisplayPort.
I won’t editorialize here other than to say what’d you expect? Apple was going to ignore the chance to sell another accessory?