Cast your minds back to the launch of the 27" iMac in mid-2009. One of the new features was Target Display Mode, which introduced the ability to use the iMac as a pure external monitor, bypassing the built-in computer entirely. This could be useful if you wanted to dock a laptop to a big screen temporarily -- or if you were worried that the substantial investment in a 27" screen would, in years to come, be hobbled by the aging CPU and GPU inside the iMac. The theory was that you could eventually just stop using everything but the display.

However, Apple has <s>quietly dropped</s> substantially modified the feature on the latest iMacs, which were announced today. For one, the Apple knowledgebase article describing the feature specifically mentions it is for "iMac (27-inch, Mid 2010), iMac (27-inch, Late 2009)," excluding the newest model. For two, while this archive of the old model's tech specs page states "27-inch models also support input from external DisplayPort sources," there is no such mention on the updated page for today's model.

UPDATE: MacRumors spoke with an Apple rep who said the feature is still supported on the new models (through the Thunderbolt ports), despite not being mentioned on the tech specs page.

UPDATE 2: Macworld has confirmation from Apple that the new iMacs will support Target Display Mode, but only when the device they are connected to is also a Thunderbolt-equipped Mac; the trick won't work with older mini DisplayPort models.

So, a new MacBook Pro connected to a new iMac will work, but older MacBook Pros will not, and a current MacBook Air won't work either. This also means the popular Kanex adaptors (which convert HDMI signals to mini DisplayPort ones, letting users plug things like game consoles into their iMacs) probably won't work with the new iMacs -- we've contacted Kanex to ask for confirmation.

This change is probably tied to the mini DisplayPort being replaced by two Thunderbolt ports. Whatever clever hack Apple did to make the mini DisplayPort connection bi-directional in the earlier iMacs is presumably difficult or impossible to replicate through Thunderbolt, as it has less control over the protocol (remember that Thunderbolt is an Intel design, and uses a special Intel communications chip).

UPDATE 3: Consider that, for legacy mini-DisplayPort devices to work, the Thunderbolt interface would need to consume a raw display signal -- not one wrapped up in the clever Thunderbolt protocol (note, PDF link), which combines PCIe and DisplayPort data into a single stream. This is probably why the new Thunderbolt ports cannot maintain backwards compatibility with older devices. However, when the device sending the signal also has Thunderbolt, they can negotiate a way to send the signal that the receiver can understand.

What do you think, readers -- was Target Display Mode always a pointless curiosity, or do you think this is a loss for the iMac's feature set? Has anyone out there used Target Display Mode in anger?