Judging by what I'm seeing, something is wrong with Google's
PR calculations as displayed in this latest public update. I have a page on one of my sites which jumped from PR0 to PR3 even though the only link to it is from a PR2 page. (There are no external links to it.) But another page on the same site is still PR2, even though there are several links to it from other PR2 and PR3 pages.
Either the algo did the calculations wrong for this site, or these are meaningless temporary fluctuations taking place during a still incomplete update.