Since Microsoft has officially denied any possibilities of having Windows Phone 7 Series on HTC HD2, Tom Codon of HTC Pedia has shown screenshots of a working Windows Phone 7 Series on an HTC HD2. This was done through a combined effort of few developers working on a team called Russian Project.
These screenshots are pretty much real and at least I could not figure out whether these were amazingly shopped or not [personally I take them as legit].
Tom further explains in his thread that it is a working OS with every functionality working except for the graphic driver which makes the device lag. It might have been one of the reason's why MS did not want to port WP7S for HD2.
Anyway, for your viewing pleasure, here are the screens.