Update 1: Looks like Xbox Live and Xbox.com have gone down again. According to the Xbox.com support page, Microsoft is aware of the issue and is currently working on a solution.
Xbox.com also appears to be having issues as it is not displaying player profiles and is loading pages slowly. Currently, Xbox.com's support page does not list the service is experiencing any issues other than TruSkill and EXP rewarding problems associated with Halo 3. We'll keep you posted as details come in.
Original Update: According to the Xbox Technical Support phone line Xbox Live is experiencing network issues that are keeping some customers from accessing the service. Microsoft is aware of the issue and is currently working on it and recommends users attempt to reconnect in one hour. Don't shoot the messenger.