The DMZ has basically been closed for over a year or more now. This is due to (currently) African Swine Flu, but in the past - any kind of "military training," presidential visit, summit, politician meeting. So, the tour companies generally still advertise it as available because they might get *one* day to do it...but as MSP said, they often will cancel the day before and offer a much less inferior tour (which you don't have to take and you can get a refund).