We're getting a lesson today in output courtesy of input from our friends at iFixit.
Apple Insider chronicled their efforts confirming that Apple's new 27" iMac can only be used as an external display for devices designed to provide DisplayPort video. You're out of luck if you're hoping it would work with equipment supporting VGA, DVI, or HDMI output.
In a follow-up to its teardown of the 27" iMac last week, iFixit said it revisited the new hardware to see if it could display high-definition video from a non-DisplayPort external source. The results of the testing indicate that Apple's stated specifications for the iMac were correct; while video input worked as expected with a 13" MacBook Pro equipped with Mini DisplayPort, all attempts to use a physical adapter dongle to supply alternative video signals to the new 27" iMac failed.
Apple has frequently used converter dongles on its notebooks in order to support multiple types of video output signaling via the same port. For example, previous notebook models provided Mini-DVI ports proprietary to Apple which could deliver both VGA and DVI outputs using the appropriate connector. These ports provided multiple signaling types over the same physical pins.
But in trying to see the glass half full, it's important to note that any DisplayPort compatibility for an Apple MacBook is already close enough to a miracle for some. And, as a result, its likely that a big fuss won't emerge in the wake of today's news.
Apple's existing MacBooks, Mac mini, Mac Pro, and the smaller new 21.5" iMac model do not support video input at all. The company's 24" LED Cinema Display is the only other device that currently supports (and only supports) DisplayPort input. The 30" Cinema Display HD only supports DVI input, but not DisplayPort.