Moments ago, the team released a system update (aka Dash update) to all Xbox Live members. Over the next few hours - or the next time you sign into Xbox LIVE after that - you’ll receive a prompt to accept this mandatory update.  If you participated in our Preview Program, you already have the update and will not be prompted.  Hit Xbox.com for a list of what the update includes.  A couple of things to point out:

• While you can browse and purchase Avatar items and full Xbox 360 games in the console with this update, the ability to add these items your Xbox 360 download queue from Xbox.com will be available this Thursday.
• Xbox LIVE integration with Twitter, Facebook and Last.FM will be available later this year.
• Zune Video Marketplace will also ship later this fall when it will replace the current Video Marketplace.

This update will be rolled out to all Xbox LIVE members around the world over the space of a few hours, so you may not get prompted for the update immediately. Thanks for your patience and have fun.

Source: Major Nelson’s Blog