...DisplayPort is a packet-based protocol that is not at all related to VGA and DVI.
DVI and HDMI still send "lines" of content for a screen over the wire, much like a cathode ray tube would draw them; it's just the signal that is fully digital unlike with VGA.
DisplayPort however is different, and basically sends pixels in little packets across the wire. That cuts down on necessary hardware on LCD monitors to do the conversion to a format an LCD panel understands. DisplayPort even supports features like multiple end-points on one physical connection, meaning that you could daisy-chain several monitors to one computer without needing more than one DisplayPort connector on the graphics card.
The reason why the cheap DP-DVI and DP-VGA and DP-HDMI adapters work with current Macs is because the graphics cards have legacy VGA/HDMI signaling hardware on board, where the "old" signal formats are simply transported over DisplayPort connectors, thus requiring only an electrical converter (= little adapter for 20 bucks).
One of the main reasons why DisplayPort was invented was because it could make LVDS, the connection that laptops use internally to connect the graphics card with the display, obsolete. It is extremely likely that the iMac's LCD panel is connected directly via DisplayPort and will not understand any signal other than DisplayPort.
Apple's documentation at the minute only specifically mentions "DisplayPort sources" for input. Unless the iMac's graphics card has input support for DVI/VGA which it then converts to a DisplayPort signal, only DisplayPort inputs will work, no matter what passive adapters you use.
Active converters, however, would do the job, but my guess is that they cause at least some amount of input lag.
Until I read this post and a few others just prior to it on this same page I was about ready to make some rude comments about the rampant technical incompetence that was being voiced in this thread. Thankfully (I hope), Wombert and a few others have finally said something of merit on this issue. Thanks.
In any case, in order for HDMI and DVI sources to work with the iMac's video input Apple will have to supply some form of support for that capability. This could have been done in one of two ways (or a combination of both).
First option, Apple could provide an active video converter that will attach between the HDMI/DVI source and the iMac to convert the HDMI/DVI signals and protocol to DisplayPort. This would be similar to the previously mentioned $150 to $200 (U.S.) converters that a few third-parties are already marketing.
Second, it should be possible for Apple to support a form of HDMI/DVI passthrough into the iMac's video input port as long as Apple has included a switchable video path within the iMac to handle HDMI or DVI. The latter would obviously add some complexity (and cost) to the iMac itself but would make the external adapters less expensive. In the simplest case, this could mean that the external adapter might be little more than a cable that offered an HDMI/DVI plug on one end and a Mini DisplayPort on the other with perhaps some simple circuitry to enable the iMac to determine when it is connected to said adapter.
However, don't confuse this second option with just any third-party cable that offers simple conversion between the physical port types. It's somewhat doubtful that those would work even if Apple had added the necessary HDMI/DVI circuitry within the iMac. If these third-party cables do work then more props to the engineers at Apple.
In any of the above cases, however, you've still got the problem of how to handle the resolution differences between the source device (DVD player, Playstation, etc.) and the native resolution on the iMac's 27" display. None of those devices will output at the 2560 by 1440 pixel resolution that is required for the display. How this can be handled I'm not sure, maybe with hardware scaling in the iMac, or letter-boxing, or with scaling in an external video converter.
Lastly, there are some consumer devices that are being designed with DisplayPort outputs. This means that the demo that was done for MacWorld may have just used a DVD player that offered native DisplayPort output. Thus, until someone can actually test one of these new iMacs with the proper adapters on a variety of devices then I'd say that all bets are off (i.e. HDMI/DVI devices may be supported or they may not).
