It seems that Microsoft have confirmed that the HTC HD2 will NOT be upgradable to Windows Phone 7. While there still might be some hope if the ROM appears on the XDA Developers web site it certainly will not be supported from Microsoft.
For Windows Phone 7 Series we are enforcing a strict set of hardware requirements to ensure a consistently great experience for end-users and developers. While we cannot confirm that WM6.X phones that satisfy those requirements will be upgradeable, every Windows Phone 7 Series device will be upgradeable with improvements and features we deliver with subsequent Windows Phone 7 Series releases. We currently do not have plans to update the HTC HD2 to Windows Phone 7 Series.