A few minutes ago, the team released a system update for the Xbox 360 over Xbox LIVE. Sometime over the next few hours when you sign in to Xbox LIVE you'll recieve a prompt to accept this mandatory update. This update contains code to 'prepare for future growth of the service.' There are no other changes or enhancements in this update…in other words no new features.
To be clear: This is not the new Xbox experience update. That will come in the Fall.