I've never been banned, but I know people who have, and who had a PR6 drop to PR2 for no explainable reason.
Also, I read people in the Newsgroups who had a high ranking for certain keywords who plunged to #700, again with no explainable reason except that Google screwing around with the algo.
Despite their multi-million dollar IPO, I think Google is decaying, and Yahoo is arising again! If only the Yahoo site wasn't such quite so whorish, it would be a great place to search!