Nothing as of yet. They don't really do beta testing for dashboard updates.
They only had a beta for the last update as they were changing the firmware on DVD drives which could have affected some people.
They weren't called betas but they're were "preview programs" for the Spring Update that allowed flash memory support, the Kinect Dash and the most recent update. I participated in all three of those and there were more that I
missed out on.
YYYEESSSSSSSSSSSSSS!!!!! Im so anxious to try out the bing integrated kinect feature. Kinect is an amazing piece of tech. ill def be signing up for the beta this fall. Major nelson will post it on his blog when the time comes around.
Another suggestion, if you have a gmail account go to google alerts:
http://www.google.com/alerts and set up an alert with the search term "beta test" xbox 360 -- I just got into the ms. splosion man beta by seeing an alert pop into my email. Generally there are forums and whatnot that see it before I ever would, and google
lets me know :)