On Tuesday, June 16th from 7:01AM UTC (12:01 AM PST) for up to 24 hours, Xbox Live will be offline for maintenance. As a result, Xbox LIVE and 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 with limited interruptions.
This service maintenance is to prepare for some of the recently announced features that will be available later this year.
Let me reiterate: When the service returns, you will not have any new functions or features. This is for scheduled maintenance only.