Why doesn't MiniDisplay out work with Thunderbolt-only Target Display Mode iMacs? I realize this is kind of an AnandTech-fu level question.

Technically speaking, why doesn't a Thunderbolt iMac work in Target Display Mode with a computer that only has MiniDisplay out?  That is, what's preventing the 0s and 1s from happily going into the iMac's Thunderbolt port, especially when the iMac's happy to let MiniDisplay port adapters carry 0s and 1s on out to a monitor?  

That is, why can the iMac's Thunderbolt port do DisplayPort monitor out and not in?  

(I realize it's not exactly the same thing, etc.  What I'm getting at is if Apple simply didn't write the drivers to push you to use Mac laptops or if there's truly a technical hurdle that made supporting mini DisplayPort devices exceptionally difficult.)

How difficult, technically, would it be to create an adapter for MiniDisplay output into "Thunderbolt output" like the iMac &amp; Target Display Mode expects?

Edit: I can find a good deal of speculation that video should work (here or here, for example), but when I see someone who actually had a DisplayPort computer try with Thunderbolt monitors, it doesn't work. And Intel's Thunderbolt graphic only shows DisplayPort going one way.  Though that doesn't necessarily preclude taking in a DisplayPort feed, I suppose.
The display port standard only supports a video signal going in one direction (source to destination). The video card can output via display port, but  handling a bidirectional connection would require extra hardware to handle the inbound signal, and isn't part of the standard.

AFAIK the iMac's target display mode is done through the PCIe layer of thunderbolt, whereas the DisplayPort layer is only supported (by the graphics card) in the outgoing direction.