Information in this article could be outdated, please read more here.
Google takes a snapshot of all internal PageRank values approximately every three months, and then exports these values to the Google Toolbar.
The snapshot is taken a few weeks before it becomes visible on the Toolbar, which may for some web pages cause a discrepancy between Live PageRank and Toolbar PageRank during an update. People who are not aware of this fact may draw the conclusion that this is evidence that Live PageRank is useless.
The lag can be proven by chosing a popular blog, say Matt Cutts' Blog, backtracking through the posts and running the URLs through the tool on this website until you find one post still at PR0, where the previous post has a PageRank higher than PR0. Somewhere between the dates for the two posts is when the snapshot was taken. Compare this date to when the Toolbar export was first reported. This info can easily be found using Google.