I think it's related to the "Sandbox Effect".
Google is ignoring, or severely discoutning, new links. So new links no longer increase PR and no longer affect the SERPs. The link doesn't start to count until a few months have passed.
This means that, in the future, the Google Toolbar will always display PR that is several months out of date in the sense that new links are not being counted, but actually it IS current if one considers that the new links are ignored for SERPs as well as for PageRank.