My mistake, I thought that iDVD could produce hi-def content from iMovie HD.
But, on the original point, Apple's DVD Player app can only play non-DRM HD disks, not any commercial movie that can be purchased. So, to say that it supports "HD DVD" is overstating it (the original post on macrumors claimed it has supported HD-DVD for a long time).
Whether full DRM support is added in Leopard remains to be seen. Some screen shots of the new Leopard preferences would probably explain a lot.