If you've got an HTC Hermes, TyTN or Cingular 8525, today's your lucky day. The guys on the XDA Dev Forums have hacked together a bunch of Windows Mobile 6 code together to make a version that works on your phone. It's very unofficial, to be sure, and is missing stuff like voice calling, PTT, Live! Mobile, Smart Dialing and Windows Mobile Update.
If you're brave and really want WM6 on your phone, give it a shot. Just don't be a afraid to get your phone a little bricked.