If you could go this direction (HDMI/DVI->DP/MDP) then people would have been connecting their "older" MB/MBP/MBA with DVI output to the 24" LED display. That type of conversion requires active hardware, not just a cable as in the opposite direction (DP/MDP->DVI/HDMI).
This may be true, with one difference: The 24" display lacks and kind of video processing hardware, like a video card or the processor. So it requires the signal to be MD, which comes from the video card in the source (Macbook with MD, etc.). What remains to be seen is if the video processors driving the 27" iMacs will do a signal conversion coming the other way. If they were engineered to do that, Apple deserves a lot of credit. One glimmer of hope is that according to iFixit.com's tear down, the signal coming in has to go through the logic board, so it's not passively going straight to the panel. What the logic board does with the input is anyone's guess. Does it go through the GPU daughterboard as well? According to iFixit, the GPU daughterboard is off the shelf and used in many other products, not Apple's design, so maybe Apple does or does not deserve credit for leaving it alone. I don't know the video cards capabilities, so that may be one place to look.
So best case scenario, the processor does the job of converting the signal (and scaling it), and a $10 Monoprice adapter will work. Worst case, Apple will sell us an active adapter (hopefully cheaper than the expensive options out there). It won't be long before we know either way. I've been checking Youtube for proof it works. I ordered the 27" iMac and the cheap adapter, but don't expect to see it for another week or so
All of that said, I may be talking out of my ass. I'm no engineer. I just hope that there is some kind of solution for imputing non-MD external video sources without outputting too much from my wallet
