There is a new spec on the table for a disc with two HD layers and 1 SD layer on the same side of the disc. Right now, the spec only supports single sided discs that have 1 HD layer, 2 HD layers, or 1 HD / 1 SD layers. Then there are double sided discs.
So, unless I'm missing something, it looks like all of the bonus material is ported over... including the 90-minute documentary (the "scrapbook"). So that is a good thing. At only $20, this may be just the second movie that I already own on DVD that I rebuy for HD DVD.
(Though I wish studios would start enhancing bonus features for widescreen, especially now that we are in the HD age. So few DVDs have done that, in fact the only one I can think of off the top of my head is the Lord of the Rings extended editions.)