Having met with some of the folks involved in these efforts at CES, I feel I can safely say nobody even knows what an HD DVD is going to be, much less is there any thought of compatability with OLD hardware. Old software, sure, but old HARDWARE?
HD DVD is most likely going to be of sufficient size and encryption complexity that it won't fit on existing media, first of all. Second, my understanding from talking ot quite a few folks in the process is that the studios (MPAA) are terrified of PC-playback, and therefor I would not expect to see widespread studio adoption until the media to video playback path is hardened (which means TCG and Longhorn).
So I wouldn't be holding my breath.
Sure, WMV9 CAN do high def, but it's irrellevant until the copyright holders we all want to buy content from sign off.
Unless you're just trying ot archive stuff, in which case, you can do that now.