http://store.steampowered.com/uiupdate/It's pretty sweet!
To try it out, you simply go to the "Settings" menu option and click on the "Change" button under "Beta participation".
A lot of people are downloading it right now so you may not be able to get it right away.
If you get a message that it cannot connect to the Steam servers to download it, you can press Enter to skip it and get the old UI. Then it will just try to download it again the next time you start Steam.
