Microsoft sent word earlier today that Xbox Live is in line for some scheduled maintenance. It'll take place next Tuesday, and could leave the service down for up to 24 hours.
The servicing will commence at 12:01 PST, and will leave Xbox-related online services down (including everything to do with Xbox Live) save for the official boards, though those will be reduced to read-only mode.
According to Microsoft's Larry Hryb, "This service maintenance is to prepare for some of the recently announced features that will be available later this year". Like Facebook, Last.fm, etc etc. Note however that it's only to prepare the service for those new features, not actually introduce them.
Xbox Live Service Maintenance [Microsoft]