Major Nelson just announced:
“I wanted to let you know that the team has 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 receive a prompt to accept this mandatory update. If you are signed into LIVE already, you may be signed out and asked to accept the update and you’ll be back online in a few moments.
There are no new features or changes in this update. This update is preparation for additional system enhancements (Facebook/Twitter/Last.FM) that will arrive later this year.
Again, there are no new features in this update. “