George Hotz, otherwise known as GeoHot has posted what he claims as the world's first PlayStation 3 root key, which is used for code signing games.
The root key enables the PS3 to know which is a genuine piece of software and which, is not. If other hackers start using it, the key will allow the use of custom software and pirated games. It will enable the use of homebrew without the need for PSjailbreak-type USB devices.
The bad news for Sony is that the key cannot be changed without hardware modifications - firmware updates are not enough to stop it. Even if it was, it would render current software inoperable.
GeoHot was also responsible for helping crack the iPhone.