Only for maintenance, hopefully it shouldn’t take too long for the service to be restored according to the message:
We’re building a better Microsoft Advertising pubCenter!
We are currently performing site maintenance. All ad units will continue to serve ads normally during this process, but you will not be able to log in.
Service should be restored by Saturday, March 12, 2011 6:00:00 PM. We apologize for the inconvenience and appreciate your patience.
For more information contact us.