Google has started targeting private blog networks that are used to boost « money site » rankings through their link building capacity.
The move has not been officially acknowledged by Google but Matt Cutts has responded via a tweet to the Black Hat SEO community:

tweet-matt-cutts

 

How will penalties be applied?

For the time being, Google is manually informing webmasters of sites concerned that a penalty will be applied due to thin content. This message is delivered through Google Webmasters Tools.

Here is a summary of the message received by these webmasters:

message-thin-content

 

The blog networks more penalized than blogs with poor content

It is in fact the blog networks that are Google’s real target. Some are finding themselves penalized despite having high quality content.