I just dropped in to check on my friends at WPW and found this thread. I still don't know whether it's real or just a figment of my imagination due to heavy medication. Just in case it's real...
1 - Google
re-calculates PR on a schedule which is known only to them, probably anywhere from once a week to once a month. Who knows, it might not even be a regular, set-in-stone timetable.
This frequent
PR re-calculation can be observed in action by closely watching the SERPS. Quite often there will be a major shake-up in the SERPS for various search terms that cannot be explained by the constant jostling for position that occurs on a daily basis.
Every one of these shake-ups drastically affects the SERPS for a handful of search terms, but not all. That's why we frequently see posts exclaiming "A major update is underway" when the SERPS for the terms we follow ourselves haven't move a bit.
2 - Google
updates PR on the Toolbar on a not-so-regular schedule (approx every 90 days or so).
When the Toobar
PR update takes place there is no noticable change in the SERPS other than the normal changes that take place daily. Why? Because the Toolbar
PR update is simply reflecting the value of the latest internal
PR re-calculation. In fact, the
PR values that Google uses as a factor in the SERP rankings has probably already changed at least twice since the last Toolbar update.
Sorry for the rather incoherent post - I'm enjoying the benefits of some quite strong pain medication.