For years and years, as far as I know, users will sporadically get a survey from Google asking them to rate the search results or ask which search result they prefer.
Eli Schwartz (no relation) emailed me a screen shot about a week ago of Google asking him to take such a survey. It asked him to "help improve Google" by telling Google "Which result do you prefer" of the search results.
Here is a screen shot:
Google sent Search Engine Land a statement on this when we asked, they said:
As you know, we always ask for user feedback in a range of forms - from live experiments to inviting people in to our UX labs - in order to improve our products. This is one of our experiments - one of many signals we take into consideration to make search better.
Again, this is nothing new, but the first time I've seen a clear screen shot of it and the first time I think we've written about it here (although, I can be wrong about that).
Forum discussion at Google+.