Hacker George Hotz, also known as GeoHot, has posted what he claims to be the PlayStation 3 root key, which is used for code signing games. GeoHot, who is known for helping crack the iPhone, has not only found the key, which groups have claimed in the past, but he has posted it to the public.
The root key lets the PS3 know that a piece of software is legitimate. If other hackers start using it (and it is indeed the real one), the key will let them make custom software or easily use pirated games. It will make it possible to run homebrew without the need for psjailbreak-style USB-devices. It also provides hope for those at firmware version 3.55 that have no way of downgrading.
The key cannot be changed without hardware modifications (firmware updates are not enough), and even doing so would render current software inoperable. In short, Sony will have a hard time fighting hacks developed with this key. The company has yet to issue a statement regarding the news; there are likely quite a number of people currently working overtime to figure out the implications of the key being available for the world to see.