According to Major Nelson, if you applied to be in the customer preview of the latest dashboard update - and you were chosen - you should have received an email that looked like the above picture. The update should be applied the next time you turn on your Xbox 360.
If you didn't get accepted, don't worry, they are still accepting applications and it looks like a 2nd batch of invites may go out.
The update includes many new features including Netflix Parties, Game Ratings, UI Enhancements and numerous other tweaks. We've got a thread going with Co-Optimus member feedback right here. If you're running the new dash, let us know what you think!