PR is updated internally at Google everyday and this factored into the rankings then. They periodically export a historical representation of that internal PR to the toolbar for us to amuse ourselves with every 3 or so months. Rankings don't change when they do that export of data as its already factored into the rankings. I did see a comment somewhere that they are doing one at the moment - I have no idea if they are as I take so little notice of it.
The last one, I think, was in mid April, the next, maybe end of this month, next month or any time before the Sun goes into the next solar fuel cycle. :-()
Seriously though, I'm pretty sure April for the last and agree with dockarl that the next will likely be in August.
Definitely agree with seo101 that internal is updated almost continuously although I haven't heard rumors about one going on now.
But then again, I'm deaf in one ear and can't hear out of the other and even were I able to hear rumors, I usually just wait until something happens before paying any attention. ;-)
Little know fact, but Matt Cutts always hides easter eggs on his site to tell you when the next PR update is, I just can't remember how he does it, exactly. It real complicated something like the 3rd and 4th letters of the 128th word of the 2nd to last post that starts with a "P" (for page) on his SEO category is the month, and then the day of the month is the numerical location of the letter "R" (for Rank) in the prior post that begins with "U" for update. Something like that, you'll have to study his blog or search for it, but the information is there. . . . . . . . . . . . . . . . . . . I'd waste about as much time on my made-up theory as worrying about PageRank updates. :)
