If a new site just got a PR 0 from being without any PR before, that PR being 0 doesn't mean a penalty right? but if an aged site drop from PR1+ to PR 0, that means a penalty. right?
it does not have to be a penalty.Drop in the backlinks count or drop in the page rank of the inbound links will also lead to drop in the page rank of your site.But remember that the PR penalty will only be given when you sell links.So if you have been selling links and the PR has dropped then most likely,it should be due to the link selling.
It doesnt mean anything ... your sites PR will always fluctuate depending on incoming back-links thx' malcolm