from: http://en.wikipedia.org/wiki/Thunderbolt_(interface)
"... Thunderbolt controllers fold data from existing DisplayPort systems with data from the PCI Express port into a single cable..."
... However, Thunderbolt is bi-directional, which often requires sophisticated reflection and cross-talk suppression techniques, such as used in 10 Gigabit Ethernet. ..."
So as it currently stands, the technology is not compatible for the Thunderbolt display to successfully communicate (bi-directional information transfer) thru the older mDP port (with is purely a video 'output').
from: http://en.wikipedia.org/wiki/Apple_Thunderbolt_Display
"... This display is only officially compatible with Macs that have the Mini DisplayPort connector or the Thunderbolt connector. A third-party converter must be used in order to use this display with older Macs. ... "
I guess, never say never... as by definition, they mention a 3rd party device would need to be used. But as it looks, there is nothing on the market to do this.