cool post with links to a variety of search research. I hope to have time to read all the referenced papers.

Greg Linden asks

The probabilities of jumping to an unconnected page in the graph rather than following a link -- and briefly suggests that this personalization vector could be determined from actual usage data.

In fact, at least to my reading, the paper seems to imply that it would be ideal for both of these -- the probability of following a link and the personalization vector's probability of jumping to a page -- to be based on actual usage data. They seem to suggest that this would yield a PageRank that would be the best estimate of searcher interest in a page.

But, if I have enough usage data to do this, can't I calculate the equivalent PageRank directly?

Ho John Lee answers Greg's question here.