Blu-Ray's extra storage capacity is really beginning to come into play. Many big Blu-Ray releases, like Spiderman 3, come with all of their extra features in 1080p. Most HD DVD releases, however, come with 480p (SD) extras.
The latest example is Warner's Harry Potter and he Order of the Phoenix. Not only does the Blu-Ray version come with an extra 40 min. special from A&E on the making of the movie, and it and many of the other special features on the disc are in 1080p. All the extras on the HD DVD version, however, are in 480p. The additional scenes and other featurettes are all in HD on Blu-Ray, but only SD on HD DVD. Sure HD DVD gets PiP and that community chat thing, but I'd much prefer getting HD extras on my HD format.
Check out the backs of the OotP cases (top right, under "Special Features", and more info in the "Special Features" box in the middle of the case):