Google has been experimenting with tons of UI changes to the SERPs. One such experiment was "Dissatisfied?" Added to Some Google Results, which now turned into a few more tests. Why are they doing this? Matt Cutts explains that "Everyone who thinks that search engines are completely perfect and can never be improved, please raise your hand."
Danny clarifies that the most recent report by ClickZ that Google Tests "Commercial" Results In Organic Listings is false. Before he was able to clarify, the buzz went through the Internet, through battellemedia and MediaPost. Gary Price informed me that Danny updated his blog entry with an official statement from Google.
Google is testing an automated technique for detecting when an alternate query might help users find what they are looking for more quickly. For these searches, which are both commercial and non-commercial in nature, Google displays one or more alternate queries together with a preview of their top results.