A scheduled Xbox Live outage is schedule for June 16 between the times of 12:01 AM to 11:59 PM PDT. Users of the service will not be able to view the "My Xbox" section of the dashboard, the Netflix movie service will be down and the official Xbox forums will be in read-only mode.
The reason for the maintenance is due to scheduled maintenance of the service, and Major Nelson aka Larry Hryb, Director of Programming for Xbox Live, "When we turn the service back on you won't see anything different on your console."
In addition, Zune services will be down during this same time. During this time, you will not be able to access the Zune Social, Zune.net forums, or Zune Marketplace.