Prior 27" iMacs have had the ability to run as external displays in Target Display Mode.
This was very nice when the view connection was DVI, but the current ThunderBolt connections limit it to other systems with ThunderBolt and the appropriate drivers. This limits it to pretty much using the display with other Macs.
My quick review of the Apple's tech spec for the new iMac doesn't say if it has this feature.
I had the worst time getting Target Display Mode to function sanely. I had thought that it was some kind of hidden systems feature or something, but at least at the time it required an Apple keyboard be attached and a specific key sequence be pressed during startup to make it happen.
On my mid 2011 iMac ⌘ + F2 enables target display mode once something is plugged in to the mini display port. Nothing needed at startup.
Not sure about having to be an Apple keyboard- probably works with any keyboard as long as you know which key is mapped to command. (Usually the "OS" key).
While you can use a DisplayPort display with a Thunderbolt iMacs, you cannot use target display mode on a Thunderbolt iMac with a DisplayPort 1.2 PC (or pre-thunderbolt mac).
This was very nice when the view connection was DVI, but the current ThunderBolt connections limit it to other systems with ThunderBolt and the appropriate drivers. This limits it to pretty much using the display with other Macs.
My quick review of the Apple's tech spec for the new iMac doesn't say if it has this feature.