Barry Welford draws everybody's attention by writing a post on Google using tracking URLs in the search results and whether it uses this for ranking purposes.
It's clearly stated that in its contextual advertising programs, Google will adjust the ordering of Adword ads based on Click-through-rates (CTRs).
I'm getting the distinct impression that the same thing is happening on Google's regular search. If a web page is near the top, then it seems as if it may gain a position or two if the title and snippet seem more appropriate in a particular keyword SERP. It would of course be only a slight extension of the way Google now is personalizing results based on what it knows of your own particular online behaviour.
A long time back Search Engine Roundtable wrote a post on Google Tracking Clicks – Any Affect on Rankings?
In all honesty, does the click data affect relevancy? You have to think so, on some level. Google is probably storing this data in order to do UI testing, relevancy testing and other kinds of testing. So on some small level, one can argue it affects relevancy and possibly a page's rank. But it is known, that clicking on your page in Google over and over again will not shoot your site up to the number one slot.
Search Engine Roundtable in another post says that since the Google Personalized Search has been altered and made more user friendly, click data becomes more significant.
Do I believe Google and other search engines use this data on a limited basis? I do. But I honestly don't believe they have a strong and direct correlation to how high a page my rank in the search results. Cre8asite moderator EGOL said that "I have no doubt that these metrics are used – and enjoy the result." But again, to what extent?
Search Engine Roundtable has a good post on it.