This feature seems new to me, Google seems to be testing an option to remove supplemental results, you may have noticed that many now give 404s. When you click it a yellow box pops up below (without refreshing the page) telling you it has been removed and there is an options showing this:
I noticed it on this search which I found on digg.
Official Google help on it can be found here!
Please tell me whether you people see the same options..