Jared Spool of User Interface Engineering posted a great article on search engine results page design and how it’s plain difficult to get it right (right = effective to support the goals). I can personally attest to the fact that a well designed SERP can greatly increase usability, i.e. the effectiveness in findability and completing the task at hand.
Enjoy the article. Waiting for Part Two. 
Very interesting post and a common problem for a lot of designers.