On Monday September 29, 2008 Xbox Live will be down for 24 hours on a scheduled maintenance. As Larry Hrub pointed out, there will be no new features when Live is up and running again. The maintenance is a preparation for the upcoming Fall update.
So on Monday, 9/29/2008 from 12:01 AM PST, or 09.01 AM European time. Check this link for your local time.