Major kudos are in order for a band of modders known as The Dark Forces Team who went and released a Windows Phone 7 ROM for HTC's HD2 handset.
You can find instructions on the XDA-Developers.com forum, which aren't terribly hard to follow. You'll need to download the Windows Development Tools and Zune PC App, and once you've done that, it's just a matter of following a handful of easy steps.
The latest hack/ROM even allows access to the Marketplace and Xbox LIVE service, at least for now. Keep in mind that this isn't an official release, so the usual disclaimers about warranties, bricked devices, and proceeding at your own risk all apply.