The Official Google Search Blog reports on 40 changes it made in February. Trying to keep up? Yikes! This one should keep people guessing:
"We often use characteristics of links to help us figure out the topic of a linked page. We have changed the way in which we evaluate links; in particular, we are turning off a method of link analysis that we used for several years. We often rearchitect or turn off parts of our scoring in order to keep our system maintainable, clean and understandable."