On Monday September 10th from 0200-0600 PT (0900–1300 GMT) Xbox Live will be offline for scheduled maintenance. As a result, the my Xbox’ section of Xbox.com will be unavailable, and the Xbox forums will be in read only mode (you won’t be able to post.) The remainder of Xbox.com will be available.

There are no new features that will appear when the service comes back online (i.e. there will not be a dashboard update waiting for you.)  Everything should be back online and ready to go by 1pm GMT.