There are enough indications that Google might have changed the ranks of SERPs. Over the past couple of days many webmasters have observed and reported that their rankings have taken a dip for some searches. The dip has been from rank 2 to rank 6.
This dip however can only be seen for specific searches terms which means that it can't be considered a penalty for the whole domain. Most probably it appears to be a backlink or a an off-site issue.
The main points of observations which have led to these conclusions are: The targeted sites are not new and have been around for some time. Considering the big search terms the rankings have been at #1 over a long period. Other searches still yield rank #1, though not always but for the most part. And off course a dip in rank from #2 to #6 has been witnessed too. Though the analysis is by no means a perfect but it does point to the trend.
The discussion continues at Webmasterworld