A few weeks back some enterprising coder managed to put a WP7S skin over his HD2's UI. Until recently, that seemed like the closest the HD2 would get to actually running Windows Phone 7. Now Akihabara News has links to two videos of WP7S working on an HTC HD2. This is still a demo, so things don't work perfectly, but this does NOT look like just a skin.
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The OS is a little bit sluggish (not excessively so) and it's clearly still in the developmental stages. That said, these videos very definitely show WP7S (or an incredible imitation) running with full functionality on an HD2. What does this mean? We have no idea at this point. HTCPedia is rather vague on just where these videos came from. Still, at least we know there is a way for the HD2 to upgrade from 6.5. That alone is great news.