It's possible; Windows Phone 7 Series shown running on an HTC HD2
The official Microsoft statement is that Windows Phone 7 Series will not be provided as an upgrade on the HTC HD2. However, developers just see that as a challenge and already have it running, several months in advance of release.
Microsoft previously revealed that the fantastic HTC HD2 would note upgradeable to Windows Phone 7 Series, but the developers never seem to let statements like this get in their way. The HTC HD2 appears to have at least the minimum specs needed to run Windows Phone 7 and according to information posted over on Engadget Windows Phone 7 has already been ported to work on the HD2. The two YouTube videos are embedded below and as you can see it does run, but appears to be a bit laggy at the moment. We still have several months until WP7 release so I imagine this is a pretty early version of the OS as well.
The folks at XDA Developers are well known for customizing and optimizing HTC Windows Mobile devices. I especially found them useful for reinstalling ROMs that removed carrier junk from devices. I am not sure if I will be upgrading my own HTC HD2 to WP7 since I really like the media and other aspects that are supported on the device running WM 6.5. I can multitask, copy and paste, and load up apps from wherever I desire on my HD2 at the moment. Also, by the time we see WP7 devices released I am sure we will have even more amazing hardware.