Slashdot ran an article Symantec Says No To Pro-Gun Sites, describing how Symantec's censorware is blacklisting pro-gun sites. Without getting into specifics, this issue is in fact very well-known, though it's resurfaced from time to time. It's been going on for at least three years, and has been covered in various places. I have a lot of information on the topic, and considered writing about it a few months ago, when it went around a few mailing-lists again. But I thought it'd be "old news".
Take a look at GunCite 's page on the topic, which started around three years ago:
Censoring the Gun Control Debate
http://www.guncite.com/gun_control_i-gear.html