Google appears to have recently updated their Page Rank for a large selection of websites. 

Nearly all of our clients have seen their Page Ranking increased in the past 3 days by 2 or 3 points.

The time it takes for Google to update its Page Ranking appears to have slowed down considerably in the past 2 years with it taking up to 6 months in some circumstances.  Many believe that Google has an internal Page Ranking system which may be far more regularly updated.