Google only knows when will it happen. But based on past scenarios, it normally updates every 2-3months, so I predict it will happen on March.
Actually, Google updates PR on a daily basis.... but only publishes it PR changes approximately every 3 months.
Why don't your read through the 84,000+ DP threads asking the same question: http://www.google.com/search?source...tp://forums.digitalpoint.com+"next+PR+update"
I have a feeling.. tonight is gona be a good night! Most likely the update will be end of March, I can't wait just to see how google will treat me this time. Last PR update was catastrophic, well not really but yes all the PR went o 0 due to implementing my new no-www policy I hope this time Google will be nice since I bought a Nexus One and been a good boy Seriously, nobody knows except the google crew.. the truth is the big G PR is over rated. Good luck all.
Google updates it's internal PR all the time, the PR Export you and many are waiting for is just a snapshot of the internal PR Values. Why don't you just forget about Toolbar PageRank and focus more on what is really important ? Traffic & Conversions are all what counts at the end of the day