There’s a good post over at http://blog.nfocus.co.uk/2010/07/oi-what-happened-to-my-test-results.html by Danny Crone that goes over a scenario where a Retention Policy was specified on a Build Definition and testers were unpleasantly surprised to find their Test Results history disappearing as older builds got cleaned up.  Check out the post to see a good walk-through on how to avoid this.