I think different sites update at different times really, I haven't seen any evidence of a giant wave of PR changes, they all seem to be random.
@pushkar,me too,still waiting for the PR update,my Alexa ranking keep climbing,now at 31k,but my PR still at 0.. well,i know,PR and Alexa rank mean nothing but yeah,it's good to have some PR and got something to talk about,lol