http://www.seobythesea.com/?p=977
Yahoo's recent patent seeks to address a few of the challenges posed by the current assumptions behind Page Rank:
1) Not all links are equal
2) Bored surfers don't go to random pages
3) Bored surfers don't only go to trusted pages
4) Sometime PR calculations cheat
5) Pages change and lose value at different rates
FYI. Enjoy!