The PageRank display on Google's tool bar, which is available for most browsers, is what so many Search Engine Optimisers and Search Engine Marketers assume to be a vital factor in the calculation of where their pages will appear in the search results for any given query and treat as the ONLY goal of their promotional link building efforts. Now PR is a part of Google's ranking algorithm, BUT it is NOT the PR as displayed on the tool bar, referred to from now on as the SGB. Which is an acronym for Silly Green Bar.
The values of PR that are used in ranking is "real" PR, the formula for which can be found at "The Anatomy of a Large-Scale Hypertextual Web Search Engine" in section 2.1.1 Description of PageRank Calculation. Take particular notice of - Note that the PageRanks form a probability distribution over web pages, so the sum of all web pages' PageRanks will be one. That means precisely what it says. There IS only a value of 1 that is shared among every URI that is in Google's index which, in 2008 was at the one million million (1,000,000,000,000) mark.
Gives the SGB numbers of 1 to 10 a different perspective at least.
Periodically the Google system converts and exports this minuscule number as an integer value in the range of 0 to 10 to the datacentres that provide the data for toolbar queries, and this is when you see a so called "PR update", Which is not really an update, because real PR is calculated and updated continuously.
In the early years of Google (1998 - 2003) PR was only recalculated once a month or so, due to the processing power required. And until December 2001 when the first Google toolbar was released, the only visible "value" of PageRank was to be found along with the "home" page listing in the Open Directory Project (ODP) also known as DMOZ (Directory Mozilla).
That is of is course assuming the site had a listing in the ODP. This bar showed a value of 0 to 8 which again bore no resemblance to the actual values of real PR. However, as you now should be able to tell, PR of either kind is not a worthwhile goal for your promotional effort, as, with only a value of one to be shared around, the value assigned to any particular URI will go down simply if more URIs are added to the index than are removed in the same period. PageRank is simply a byproduct of your "link building" not a measurement of "how well" it is going.