Starting September 29th at 12:01 AM PST, the XBOX Live service will be down for at least 24 hours. Microsoft assured everyone this down time is strictly for maintenance issues.
The "My Xbox" section of xbox.com will also be down as all the Xbox forums will be in "Read Only" mode.
It has been rumored that it is to test out and/or prepare for its "New Xbox Experience" update that is coming later this fall. The new Xbox Experience promises to use Live to fundamentally change the console.
It will be introducing an Avatar system, new social features, a mass market-bating features plus much more.
When the XBOX LIVE service comes back online, do not expect any new features.