This happened to me and it was back to normal in a few days. Some weird stuff is always going on with Google. I've learned not to let it get to me.
same thing happened with my site Caltweet.com but now it returned it had a small affect on our search traffic..not a bighit though.
You need backlinks to get the PR back. Work on it. I suggest you to join some forums and add sign. there.
yes its come back but is now at 2 so not sure why. I have started to delete a load of links in my comments which are not related to my blog content and also placed a note next to my comment box that I will be doing this from now on. Thanks for everyones support