After much to-ing and fro-ing, the definitive answer as to whether the HTC HD2 can run Windows Phone 7 or not is in - it can. HTCPedia forum-dweller Tom Codon has successfully installed the port on his HD2.
It's not a perfect port however, as Codon has reported it's a bit laggy (evidenced in the videos below), but at least the GPS, Wi-Fi and Bluetooth is working successfully. Sadly, Microsoft has already stated they won't be offering WP7 as an update to existing handsets, so installing Codon's ROM is likely the only way to do so. [HTCPedia via Redmond Pie via BGR]
UPDATE: Or, it could all be fake. What do you think?