tedster - 4:11 pm on Jan 11, 2011 (gmt 0)

PR that affects rankings is calculated internally by Google - and on a continuous basis. that's what affects rankings, not toolbar updates to the publicly visible PR.

most of the reported updates since last April have been sporadic.

Yes - and that's getting more and more mysterious as time goes on. Google's JohnMu recently gave us a hint that a more widespread update may be coming soon:

Given the time since the last update, I imagine you'll start seeing another one in the near future. [google.com...]