Enquire, Did-it, and Eyetools recently issued a joint study about how searchers interact with Google search results with some very interesting results.
This study reveals that generally searchers look at a Google search results page in a "F" shaped pattern: top left first, then across the top to the top sponsored listings to the right, then gradually down the page and to some degree across.
From here the report digs into more detailed and complex findings. One of particular note is the idea of "semantic mapping," which basically asserts that searchers don't necessarily look for top results based on frequency of keyword use but instead seek a connection between a given listing (keyword phrases included) and a concept in his or her mind.
What does this mean to you? Well, it is very important to think beyond a simple keyword phrase and consider what concept does a keyword generally create in a searcher's mind? By thinking more about the intent behind a keyword search, optimization can be improved to not just focus on keyword density, but to place keywords in a content that matches a searcher's concept.