The Federal Trade Regulation announced this week that they will begin applying guidance for Use of Endorsements and Testimonials in Advertising to social media such as blogging.
Here's a quick video that gives a simple summary:
Long story short: If you get free stuff and write about it, say so.
Disclaimer: Neither Mary Engle, nor the FTC, has provided compensation, benefits, free samples, or sexual favors in exchange for posting this entry.