A few of our competitors in various categories with PR's ranging from 6 to 8 on their homepages just went to PR0 on their homepages (possibly for buying links). Their subpages retain their PR.
My home page went to PR0 this time and subpages hold their PR. I haven't bought any links or done anything tricky. Clean site (as far as I know). I'm not worried so far because I haven't lost any rankings or traffic.
Aha. I just checked now and my PR is back. Actually went up. But the point being that PR0 doesn't necessarily mean penalty.