Google uses technology in Adwords to rank based on ctr. they have tracking urls in the natural search results, so it wouldn't take much to do the same in the these results. They certainly seem to do this when you log into Google and do a search - they know what sites you have visited and ranking are different to the ones resturned when you are not logged on. So does CTR affect rankings for a given term? Not sure how you would go about testing this.