I took the USO tour, which left from the army camp, Yongsan, just down the street from Itaewon.

Can't speak for what the 'commercial' tour companies do, but having been on this USO tour, it takes you not only right into the DMZ, you get to actually walk to the other side of the table in the conference building.

The pre-tour briefing just short of the DMZ at Camp Bonifas is very enlightening, humbling, and interesting, this is where you sign the waiver. This is where they tell you about the history of the war, the DMZ, and the infamous tree chopping incident.

A microphone/speaker wire demarcs the border, so you definitely have been in North Korea on this tour.

A guard, who looks quite a bit more than fit, (South Korean) blocks the back door in case any tourist is thinking of defecting to North Korea. Having been in the room, I'd place heavy bets you wouldn't get past this guy.

Other points on this tour are to an observation post where you can also see into North Korea which also overlooks the area where the actual tree incident took place.