You know that traffic doesn’t perform the same every day of the week, which is why you should make your tests last several days.
But had you thought that one version may be better at certain times while another version better the rest of the time?
Look at this split test:
http://whichtestwon.com/banner-ad-lp-test?pollid=74
Makes you want to break down your tests by day and time and after getting the results, have a script serve the winning versions based on that.