In the next 4-6 weeks, we plan to release FBJS2 to the live site, pending the feedback we receive. Once FBJS2 is live, it will replace the current version of FBJS. Again, you don't have to change your code at all, but we want you to make sure everything you have works correctly with the new version (i.e. we haven't broken anything standard or non-standard that used to work). We have done rigorous testing on this already, but we strongly encourage you to test your applications in the FBJS2 sandbox as well to ensure that they will continue to work and highlight any bugs to us.
To view the sandbox, visit http://www.fbjs.facebook.com. Simply go to your application and it will be running under FBJS2.