In case you were wondering what the system update addressed today, according to Major Nelson it simply fixed some "interruptions" with certain games on Xbox LIVE and nothing else.
"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 receive a prompt to accept this mandatory update. This update addresses interruption issues that some Xbox LIVE users may have experienced when playing a limited number of Xbox 360 games.
There are no other changes or enhancements in this update…in other words no new features.
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."