A somewhat hard to read thread at WebmasterWorld describes Google as testing showing a related keywords type of box in the position where Google would show their AdWords search ads.
From what I understand is that the initial query was not commercial enough to drive search ads. Google offered up related keywords that seemed to be more commercially oriented. If someone refines their search by clicking on those related keywords, Google would then show an search ad for that commercial intended query.
Tedster, the WebmasterWorld administrator, tries to make heads and tails of this as well. He said:
So in the usual Adwords position, instead of ads you saw links to related Google searches -- searches that happen to have commercial results. In other words, those "related" links themselves aren't ads at all except for the fact that Adwords might be present on the SERP you get if you do click.
Google tests a lot of things - sounds like a rather complex one.
I cannot find it now, but I believe a long time ago, I do remember seeing screen captures of related keywords shown above and to the right of the search results, above the search ads.
Forum discussion at WebmasterWorld.
Update: I posted a picture of this over here, pretty interesting, indeed.