Microsoft has sent word that both Xbox Live and Xbox.com will be taken offline on September 29 for scheduled maintenance. The service will experience downtime for a whole 24 hours, as Microsoft is using the time to prepare for the impending launch of the New Xbox Experience, which if you haven't heard by now, is a fully redesigned dashboard interface complete with support for Mii-esque avatars.
Anyways, if you find yourself unable to log-on to Live next week, now you know why.