Lee Odden spotted Google testing this very unusual user interface. In short, some of his searches are displaying Google results that are grouped by category. I used Lee’s screen capture and cropped it a bit to fit here.
As you can see, on the top right, you have the ability to “Turn off Categories for these results.”
As you look down the search results, you first notice headings. The first heading is “comparison shopping,” which shows two sites that appear to be in the comparison shopping area. Then you see a link to “More comparison shopping” results, which Lee explains takes you to a more: operator like search (i.e. more:comparison_shopping).
I am personally not able to see this in action.
Lee has a full screen capture at Flickr.
Postscript: Seems like Google Blogoscoped found this first, sorry for missing that.