You can sign up for the preview program on Major Nelsons's blog here. The blog entry also contains details of the new update. You'll be able to play around with the new Kinect controls, better Facebook integration, and the new "beacon" function (Allowing you to send mass invites to all your friends. That totally doesn't sound annoying at all!). The feature i can't wait for is cloud saves. You can back up your Xbox Live profile and game saves to the cloud, and easily access them from any console. The perfect weapon against forgetting your damn memory card or flash drive.
The new update should be completely compatible with the older versions of the dashboard. So, engaging in party chat with friends who don't have the new dashboard yet should still work seamlessly (should).
The date for the preview program hasn't been announced yet, but you should be notified via email when it does (if you signed up).