July 18th 2011 3:30 pm PT
Sometime during the day tomorrow (Tuesday, July 19th) when you sign in to Xbox LIVE you’ll receive a prompt to accept a mandatory system 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 that will show up once your system has been updated.
This update is preparation for additional system enhancements coming to Xbox 360 later this year.