Click through rate in natural search could be a factor in Google's secret-sauce ranking algorithm. SEOmoz estimates click through to account for 7% according to its Search Engine Ranking Factors report:
Note that SEOmoz' research is based on the expert opinions of some of the world's top SEOs, not of Google itself.
It makes sense that Google would consider click through rate as a sign of a web result being relevant to a search term. (Have you marketed yourself well in meta description tags?) Click through rates by human beings add that personal element to its computerized intelligence.
Another clue to the quality / relevance of a search result is the bounce rate, which is easy for Google to calculate the time elapsed from the click through to the click back.
A campaign such as what Rishi proposes is brilliant. His hypothetical email goes like this:
Subject: Search and win
1. Google gourmet chocolate popcorn.
2. Hunt down kukuruza.com.
3. Instantly take 5% off all our 27 flavors.
4. Promo ends 1/29/10.
Not only does this attract click through, but encourages customers to at least browse the site for something they want (27 flavors of chocolate popcorn -- this I gotta see!).
Of course, the promotion could be reinforced through content spaces (banner images) throughout the site and the promo code auto-applied through targeted selling, writing a rule that all referrals from Google for the target keyword qualifies for the promotion in the cart.
Anyone going to give this trick a try?