At one point or another today (July 19th), after signing in to Xbox LIVE, players will receive a prompt to accept a mandatory system update.
Players signed into LIVE already might get kicked out of the servers, signed out, and asked to accept the update. Only after this shall they go back online.
This update only fixes security holes, and includes no new features according to our source, once the system has been updated.
More updates shall follow throughout the rest of the year.