Xbox.com, and misrosoft stated Before the Xbox 360 Open BETA, that they were shutting down Xbox Live for a 24 HOUR Period. you cannot go to xbox.com at all until tomorrow.

PLEASE note that the beta does not end untill 12 A.M. on the 19th, we have the entire day of the 18 to play it. (although im working)

I hope this information helped.