See Where You Rank When People Click
Andre at Yoast.com has written a very interesting blog post on using Google Analytics to track the actual search engine result positions which result in clicks to your site.
Quoting from Track SEO rankings and Sitelinks with Google Analytics II:
The great thing about this new version is that it makes Google Analytics capable of tracking the clicked position. Yes you heard what I say: the position. Where the 'old' Google only allowed us to track the page a keyword was on, the new Google allows us to track the exact position.
This looks awesome and is definitely on my ToDo list when I get time to re-do my GA setup.