A one way street
According to Apple's stated specifications however, the 27" iMac's video input feature will only ever work with DisplayPort devices, and no physical adapter will change that fact.
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.
Apple's modern machines similarly all supply a Mini DisplayPort connector (originally designed by Apple but now part of the official DisplayPort specification); using the right connector, users can extract and output any video signal type supported by the computer, including VGA, DVI, HDMI, and DisplayPort.
VGA is analog video; DVI and HDMI are both digital, electrically compatible, serial video data formats that only differ in their physical connectors; DisplayPort is an entirely new format that uses a packet signaling format.
The iMac's Mini DisplayPort supports output of all three, but can only input and display DisplayPort video. Unlike moving from DVI to HDMI, converting a DVI signal to DisplayPort requires more than a cheap physical dongle; it would necessitate a relatively expensive converter box to process the signal into a completely new format and possibly also a scaler to match the output device to the 27" iMac's enormous resolution of 2560x1440.
This prevents the new iMac from serving as an HDTV-style output source for older DVI-based computers or HDMI-output devices such as the Playstation 3, Xbox 360, Apple TV, or standard DVD and Blu-Ray players. Future devices that support the DisplayPort standard will work, of course.
