First, encryption keys are not subject to copyright in the first place, so it makes no sense to say they are "now" public domain. For the sake of argument, though, let's assume that they were subject to copyright, so we can go on.
Second, a copyrighted work does not enter the public domain by being published in a court filing or record.
Court records in the US that are not sealed are presumptively open to public access and copying. While this may be technically different from a work being "in the public domain", in practice, especially if the goal is secrecy, it doesn't make much difference.
My favorite example of a copyrighted work in a public court record comes from Campbell v. Acuff Rose Music, 510 U.S. 569 (1994). The Supreme Court decision includes the full lyrics of Roy Orbison's "Pretty Woman". http://www.law.cornell.edu/supct/html/92-1292.ZO.html
I guarantee that Cornell didn't bother to ask Roy Orbison's heirs/assigns for permission to include the Pretty Woman lyrics on that site...
kb
http://gamepolitics.com/2007/09/26/judge-spanks-jack-thompso...
When a copyrighted work is reproduced in government documents that are available to the public (such as court records), it is considered fair use. A quick search doesn't turn up much specifically on this, but there is some relevant discussion on the similar issue of copyrighted works released by a government agency in response to Freedom of Information Act requests here: http://www.justice.gov/oip/foia_updates/Vol_IV_4/page3.htm.
However, it is bloody stupid to put something which you wish to remain secret in the public record, which is what I believe the submitter meant. I do hope that Sony's lawyers have recognised that this horse has bolted.
For bruteforcing the remainder of the AES256 loader key (erk), as proposed by another commenter: There are still 13 hex-digits left (e.g. 52 bits), that might take a while ;-).
http://en.wikipedia.org/wiki/Deep_Crack
AES256 is more complex than DES, but it's still in the ballpark of "possible".
