H-264, MPEG-2 and WMV are all part of the HD-DVD/Blu-ray specifications.
Oddly, DVD SP 4 incorporated a sort of "preliminary" support for HD-DVD, but Apple as a company is a member of the Blu-Ray alliance. Go figure...
Since the specs for HD-DVD is not complete, the way DVD SP 4 supports it is: You can use H-264 or MPEG-2 encoded HD streams, but the interactity and navigation features are the ones in standard DVD. Since 480i is also part of the specs, you could use not just HD, but also throw an unbelievable amount of amazing quality H.264 SD content on one of those disks.
Apple DVD Player is so far one of the only players that supports HD playback, but I believe Interactual just announced an HD capable version of their player for Windows. There are no "real" (hardware) players on the market AFAIK. Several Blu-Ray players are coming out in early 2006. Samsung announced they working on a "combo" player, compatible with both Blu-Ray and HD-DVD.
Adolfo Rozenfeld
Buenos Aires - Argentina
www.adolforozenfeld.com