In yesterday's video from Google's Matt Cutts, Matt summarized three topics that aren't necessarily new but often confused by folks in the webmaster and SEO community.
In short, he said:
(1) PageRank is not a measure of popularity
(2) Topical PageRank may help determine if a specific site is a good match for a specific query
(3) Google has algorithm changes they are working on to improve their understanding of who is an authority
Here is the video:
On PageRank not being a measure of authority, that makes sense to us but not all. Matt's example is key, porn sites are way more popular than government sites in terms of traffic and usage. But no one links to porn sites and thus their PageRank is not as high as government sites, which everyone links to.
On topical PageRank, that also makes a lot of sense and most SEOs get this. This is why anchor text was/is so important and likely more important than raw links. I won't go into this more, I am tired.
Finally, Matt mentioned again, Google is working on an algorithm for the authority in the space.