The rumors floating around the SEO industry right now is that Google has crushed yet another link network. SAPE links, a link network I honestly never heard of, was the target this time. Any of those using SAPE links were supposedly penalized and downgraded in their rankings over the past 24 hours or so.
We have dozens of threads about downgrades of Google rankings but the black hat SEO forums all seem to point at SAPE Links as the cause.
However, others are not sure if it is specific to SAPE or some other link network.
There is clearly something going on, it doesn't seem to be an algorithmic update but it does seem to be a targeted penalty attacking specific link networks and the sites/webmasters that use them.
A tweet from about a month ago from Google's Matt Cutts kind of supports this. SAPE Links is a Russian link network: