Google has added "low-quality guest blog posts" as an example of scraped content in the little or no original content Webmaster guideline. The search engine has been dealing with the use of guest blogging and posting for link building, specifically after Matt Cutts said that guest blogging is done for SEO purpose.
It has penalized several guest blogging networks and is targeting low-quality guest blogs that aim at manipulating the search results. Here's a screenshot of guidelines before the update:
The screenshot after the updateGoogle Webmaster Guidelines Updated: Considers Low Quality Guest Blogs as Little or No Content!,