On Monday April 21st from 0200-1000 PT Xbox Live will be offline for 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.
This is for maintenance, there are no new features on your that will appear on your Xbox 360 console or Xbox.com when the service comes back online. This is server maintenance only.