Cornwall at ThreadWatch has the scoop on New Duplicate Content Filter at Google? He reports on a really long WebmasterWorld thread that I wasn't yet able to read. In that thread he reports Caveman posting in msg # 171;
There are cases where established site homepages and subpages are holding their ranking for one phrase, but dropping out of the SERP's for another closely related phrase (when the site previously ranked for both) ... and where there is no evidence of dup content filters playing a role where pages dropped out.
They've tweaked something else IMO. Possibly related to linking/anchor text/kw patterns.
I have been receiving emails from people about these types of changes, but I have not reported on it until now.