There's a growing thread at digitalpoint.com regarding a probably PR update going on.

Google has PR updates every 3-4 months and I expect the PR update either June or July. This may be the start but I still haven't seen any actual PR changes so I'll be Monitoring this on the next couple of weeks and advise if there were already some "proof" of a PR update.

Some people are seeing changes in PR then after a few hours or a day, it was gone.