I don't think there's much net activity in North Korea, to be honest. You might be thinking of China? In any way filters are usually working at the ISP level, not on client machines. See http://www.boingboing.net/2008/05/22/cisco-internal-memo.htm...

That and voluntary censorship/collaboration by companies:

- Google censors itself for China: http://news.bbc.co.uk/1/hi/technology/4645596.stm

- Yahoo 'helped jail China writer': http://news.bbc.co.uk/2/hi/asia-pacific/4221538.stm

Yeah, I'm embarrassed to say that I replied before I really thought this one through. Of course they wouldn't let computers that deal with passports/customs access the web, NK or otherwise.