Netflix is remaining rather quiet as to the reason why the service went down but, speculations are that a database upgrade malfunctioned (Sounds like a cover up to me...some joe blow probably got fired and took out some revenge on their software probably...)
Due to the outage, Netflix was unable to track on incoming returns and DVD's mailed out. Of course they said "Netflix was safeguarding that such an occurrence — the second major disruption this year — wouldn't’t happen again" but we will see...