It seems that they intend to enforce the rule.
"One final comment dealing with enforcement of the proposed rule
suggested that any visitor with a photographic device on their person
would be turned away and that overzealous security guards might subject
visitors to harassment or bodily harm. NARA can assure this commenter
that those hypothetical behaviors and policies will not happen.
Visitors with photographic devices will be allowed to enter the
building with their cameras, cell phones, and other photographic
equipment. However, they will be met by appropriate signage and
security personnel throughout the NAE to explain the ``no photography''
rule. In the event that a visitor makes the mistake of displaying or
attempting to use a photographic device, they would first be warned
that such behavior is not allowed. If, after they have received a
warning, they continue to ignore the ``no photography'' rule they will
be politely escorted from the building.
http://edocket.access.gpo.gov/2010/2010-1331.htm
If people will not obey 'No Flash' rules, what makes you think they will obey a ban on cameras?