In the making-of video documentary on the 2006 DVD release of the film (Enemy of the State), it is revealed that “The Jar”, the copper-mesh Faraday cage that surrounds Brill’s workstation, was so effective in blocking transmissions as accurately depicted that audio technicians were forced to put the receivers for Hackman’s and Smith’s wireless microphones inside the cage.

also, interestingly, the DOB of the ‘bad guy’ in the film is 9-11

http://www.abovetopsecret.com/forum/thread375795/pg1

Plus,

if you watch Enemy of the State and The Conversation at the same time, secret messages from Harry Seldon get beamed into your head by a pink laser