All kinds of information is now available about Google's page ranking algorithm. All made public via the US Patent system. In fact, Google has filed a US Patent (Application #20050071741 on March 31, 2005 if you want to go look it up) that exposes all their secrets.
Well, sort of exposes their secrets. It's hard to determine exactly which of these techniques are actually being used, and how specifically they are applied. Furthermore, there are likely to be additional techniques that Google hasn't chosen to make public in the patent application.
Slashdot is pointing to a
Buzzle article about the application, where you can read all about it in a more friendly format than in the formal document.
Might be worth checking out if you want to optimize your own site for search engines, or are just curious how it all works.