Unfortunately, that's the sad state of affairs when a super power allows an overly aggressive and vague law, in this case the DMCA, to be made. This law effectively allows any organisation to "own" silly things like a 64 bit number or a word used in certain contexts - all they need to do is prove that this number is of limited use and circumvents some form of usage limitation/DRM and Bob's your auntie, this number can't be published (well, at least in the US).
The thing I find amazing is that the AACS-LA (the entity that licenses AACS to makers of HD-DVD players) didn't learn from the DeCSS debacle and insisted on issuing take down notices. This works fine for ISPs, but people will just go out of their way to make the key available by other means, like having it printed on a T-Shirt, or even by using the digits in some other form like this lovely "rainbow":
I've not explicitly published the key here - I've just printed a row of colourful boxes.
I'm just waiting for the day some large media corporation does something stupid like use the word "movie" or "dvd" to encrypt a DVD and the press find out. I know it won't happen in such a simple form, but anything is possible these days.
Update: Apparently take down notices have been sent to many sites that the display the key. Well, it seems the MPAA site will display the key if you enter them in a query on their search page.