anallawalla - 9:31 pm on Oct 8, 2008 (gmt 0)

How about old sites with tons of organic inbound links, that get suddenley downgraded from PR 4 to PR 0 ?

"Tons" is the clue. Perhaps they were sitewide links somewhere and that site has removed the links. Perhaps they were forum signatures and old posts have been archived/removed.

If they were tons of unique links from individual sites, it's an algo catching up with a particular linking practice. For example, a thin affiliate site with a thousand IBLs is "probably" a self-initiated linking pattern.

Slowly, Google seems to be catching up with abnormal linking patterns.