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.