So, here is a thought: number of pages known by google stayed constant; pagerank stayed constant; yet for some specific multi-keyword phrases inner pages describing specific widgets have dropped deep down the serps in my case. In some cases pages with a PR2 is way below PR0 pages in SERPs.
So, the only thing that's changed was the display order of search results.
And the best I can tell, it's now sorted on the PR of the site's front page, not on the PR or the page featured in the search results.
A silly irrelevant page on a front-page-PR8 site ranks higher than a good relevant end-page-PR3 page on a front-page-PR5 site.
Do I make sense? Can you check if it holds true in your industry?