NASA Says It Cannot Advertise - and Then Buys Advertisements, (NASA Watch posting referenced in this NASA Advisory Council Chart).
Keith's 25 August 2011 note: I have lost count how many times people at NASA have told me that they cannot self-promote, advertise, lobby, or otherwise try to use standard marketing tools to inform the public of the things that they do. They always cite dire Congressional prohibitions against such activities. Then they go off and totally violate these prohibitions with advertising procurements such as this one. I am not certain that they actually know what it is they are allowed or not allowed to do and just throw this answer out when they do not want to do something.
Keith's update: This story has not changed. If you ask, NASA tells you that it cannot advertise. And then they do.