Seems like it's down again now and has been down for at least the last 15 or so minutes.
Haven't tried actually booking a ticket, but logging in to My Account fails with an error message:
Our web server encountered an internal error. It was logged to aid our staff in finding a solution.
For guidance with this transaction, call Customer Care at 1-800-654-5669, Monday through Friday, between 8:00 a.m. and 6:00 p.m. (PT)
Server Name: Q19
Server Time: 12/9/2010 10:45:25 PM