office365 uses the same nameservers. Your isp must have the addresses cached. We run our own dns and don't have it cached. So I can't get anything out of anything that uses msft.net nameservers (xbox.com, microsoft.com office365.com etc.)
An employee at the Microsoft Store support line said that they were undergoing a system update. I wonder if that is connected. Either way, bad choice of a date for a system update considering the official Xbox One launch tonight.