09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0 is a block of code written in hexadecimal that has been discovered to be a way to decrypt HD-DVDs, allowing people to modify and alter the content stored on the DVD.
On February 12, 2007, Arnezami, a user of the Doom9 forums, posted the aforementioned code that could be used to break the encryption on HD-DVDs. Shortly afterward, Wired published an article on the new-found key. It was then thrust into the limelight, with many websites and blogs weighing in on the situation.
Digg was the main player in the spread of the story, and the AACS issued cease and desists to Digg, Google, and many other websites in an effort to prevent further spread of the exploit. They even claimed that simply saying the number out loud or posting it on the internet was illegal. Digg and Google complied with the AACS cease and desist order and censored the number. Shortly after censoring their users, Digg founder Kevin Rose announced that they would no longer be censoring the number. They accepted the risks of dealing with the AACS and let the users post freely, avoiding the risks of user revolt.
The following history of the original crack which led to the Digg post is outlined by Cory Doctorow:
Arnezami, a hacker on the Doom9 forum, has published a crack for extracting the "processing key" from a high-def DVD player. This key can be used to gain access to every single Blu-Ray and HD-DVD disc.
Previously, another Doom9 user called Muslix64 had broken both Blu-Ray and HD-DVD by extracting the "volume keys" for each disc, a cumbersome process. This break builds on Muslix64's work but extends it -- now you can break all AACS-locked discs.
AACS took years to develop, and it has been broken in weeks. The developers spent billions, the hackers spent pennies.
 Techmeme – Digg This: 09-f9-11-02-9d-74-e3-5b-d8-41-56-c5- 63-56-88-c0 / 5-2-2007
 Monopolies of the Mind – Google AdSense has censored me because of the HD-DVD key / 2-15-2008
 Digg Blog – Digg This: 09-f9-11-02-9d-74-e3-5b-d8-41-56-c5-63-56-88-c0 / 5-1-2007
 Boing Boing – Blu-Ray AND HD-DVD broken – processing keys extracted