Amazon appears to have redesigned their website and is doing some limited testing. I briefly saw the new UI at 7:18 PM EDT and quickly lost it. After using the site non-stop for an hour to try to get it back I turned to Twitter and saw that Nathan Corsen was among the first to see it and he sent me a screenshot of the new version.
You can see it is much cleaner and is easier on the eyes with more white space and less blue. The left navigation panel has moved into the page as well. I like it! It seems to come and go fast so enjoy while you can.