Xbox Live and Xbox.com Scheduled Maintenance [27/03/07]
Xbox Live, Xbox.com and the Xbox forums will be undergoing scheduled maintenance on Tuesday March 27th from 0000 to 1400 (PT). As a result of this maintenance, Xbox Live, Xbox.com and the Xbox forums will be unavailable during this time. You will see a "We're down for maintenance" page on Xbox.com and a similar message on your console.
Source
http://www.xbox.com/en-US/community/news/2007/0322-xboxoffline.htm