One of the most controversial aspects of HD DVD has been the included DRM that, among other things, attempts to prevent copying and playback on “non-trustworthy” hardware. That hardware includes older displays that don't support the newer DRM formats, and HD DVD goes as far as preventing playback altogether on these displays. Supposedly, however, even before HD DVD has really hit off, the DRM has been cracked. There are stories floating around all over about how, why and who, though very little has been established yet:

"Muslix64" claims to have completely backed up an HD DVD movie and provides a Rapidshare link to downloading BackupHDDVD, a small Java-based utility that aids users to backup their HD DVDs. A YouTube link to see the program in action has also been posted.
The process isn't entirely fluid, and requires users to find their own encryption keys, meaning casual users will probably shy away from it. The similarities between HD DVD and Blu-ray DRM mean that if this crack actually functions the way it is supposed to, it may be a simple matter for it to be carried over. I'm very interested in finding out how founded these claims are.