If you've been in the preview program, this might not be that exciting to you (it might not even be that exciting to you anyway). The new 360 dashboard update is being released officially on the 6th of December and will carry a few changes.
Bing will be added to the console allowing people to use Kinect to simply say the title of a game or movie and have the Xbox find it. The dashboard will also have some aesthetic changes. It will embody Microsoft's new "Metro" design and the Kinect Hub and standard dashboard are one and the same now. There are a few other random changes (like being able to post achievements to facebook) but this update doesn't sound as content heavy as some of the dashboard updates in the past.
I was also under the impression this update would let you save your profile to the cloud, allowing you to avoid recovering your account at a friend's house. However, I'm not sure if Microsoft will follow up on that. We'll find out on December 6th!