"As I mentioned earlier in the week, tomorrow (Monday, 9/29/2008) from 12:01 AM PST for up to 24 hours, Xbox Live will be offline for maintenance.
When the service returns, you will not have any new features and you will not have the new Xbox experience update. That will happen later this Fall."
On 9/30/08 @ 2:18 CST MajorNelson provided the following update:
Edit: The LIVE maintenance is taking longer thank expected. I'll have an update at 0200 PT.