If the site loses it's PR, it doesn't always mean it is a PR update. It is more likely to be a visible PR penalty (and it can happen even outside of a main PR update). Google's penalizing sites, especially those selling links that pass PageRank.
Well, well before the last PR update many of us have observed that Google is now regularly penalizing the websites for selling links but they are not doing PR changes for the websites with increased or same PR as frequently as they are penalizing....
thank you.. hope i will not lose my PR.. i don't have too high Pr and only have Pr2 for niessuh.com I got PR0 for my other blog since it is new and I hope it would get higher PR.. i do have a friend which has a pr4 before and lately he lose it and google gave his site a pr0.. so sad for his part..