I just noticed this weekend that my blogs have updated PageRank. The Google PageRank update usually happens every quarter so expect another PageRank update in October. Google PageRank however can fluctuate in real-time, and is only updated so the general public can see it on a quarterly basis.
PageRank is calculated on the quantity (not quality) of links pointing to a Web page. Approximately 100 links will get you a PageRank 2, 1,000 links will get you a 3-4 PageRank, 10,000 links PageRank 5, 100,000 links PageRank 5-6, etc.
PageRank can be an indicator of success or failure in the Google engine. If you suddenly see a major drop in PageRank the website is probably doing something it shouldn't (spam or paid text links) and Google is momentarily penalizing your website. Rectify this situation and submit a Reconsideration Request in Google Webmaster Tools.