I add pages pretty much everyday and from what I can tell the cut off for my sites was May 21st. Well, somewhere between May 21st and May 24th... I took a couple days off in there. All pages prior to the 21st are showing PR, and 24th and after are not. However, it'll also depend on your crawl schedule, but I think the end of May to beginning of June is a good estimate on how far back they went this time. Much better than that pathetic PR update last time!