- Google can detect the general theme of a site - not always accurately, but still to a certain degree.
- Google can determine relevancy of a page for a given site to a specific term (they do it for every search performed).
- Google is now public and has a huge budget for computational power.
- Google updates less frequently than before, thus allowing for more computations to be performed before updates.
- Google caches entire pages.
- Google can determine stemmed keywords.
Now, does anyone sincerely believe that Google considering the theme of sites linking to you is unlikely?
Even as a very basic example, if page A links to page B:
- Determine all words with density x% or greater on Page A.
- Determine all words with density y% or greater on Page B.
- If Page A does not contain at least z% of words contained on Page B or words stemmed therfrom, Page A is not on-topic and therefore the link is probably a manipulated link and should therefore be discounted.
Extremely basic and of course not accurate, but when you have as much data as Google has, you can play around with methods until you find a variation or a combination of similar theories that produce enhancements to rankings.
On a site or page basis, I don't know - but I bet they do!
Of course, PR (as in the decimal number we see) is probably unaffected - but ask any SEO these days and you'll hear that PR doesn't mean even as fraction as much as it used to say 3 years ago, so even though your PR goes up - "ranking power" may not.
And finally, I'm not saying the DO this, but it's certainly within the realm of possibility. I mean the competition is getting stronger, they have to explore new things!