A unique living relic of the Cold War era, Panmunjeom is a small village that happened to lie at the final battle front of the Korean War. The truce that ended hostilities was signed here in 1953, but as peace was never agreed to, the two sides are still officially at war over fifty years later and a million men stand guard in the singularly inappropriately named Demilitarized Zone (DMZ). While there are no troops in the zone itself, both sides of the 4-kilometer strip of land separating the Koreas are the most heavily armed in the world: Pillboxes, land mines, barbed wire, and tank stoppers line the entire border and stretch back halfway to Seoul in the South and Pyongyang in the North.
One kilometer east of the former village (now deserted) is the Joint Security Area (JSA), an almost circular patch of land with an 800-meter diameter. The area is jointly policed by the South and North, and the two sides occasionally meet for discussions (or gunshots). Most of the time the soldiers glare at each other across the border and have not been allowed to cross the demarcation line into each other's side since the so-called Axe Murder Incident in 1976 (see below).
I can't speak for all tour companies, but as of June there are companies that are running the tour to Panmunjeom again.
From the South
Visits to Panmunjeom from the South Korean side must be arranged in advance as part of an organized tour, although for foreigners three days notice usually suffices. Many companies advertise "daily" tours, but generally tours to the DMZ run only certain days, so check in advance. On tour days, depending on the day, various combinations of tours to 3rd tunnel, tours to Panmunjeom, and joint tours are available.
The rules for who can enter are set by the UN and the US, not North Korea. At present, citizens of China and and South Korea will need to make arrangements well in advance (at least two months), while citizens of Afghanistan, Pakistan, Cuba, Iraq, Iran, the DPRK, Sudan, Syria and Libya cannot participate. Note that Hong Kong/Macau SAR passports are considered "Chinese", so use a British National (Overseas) or Portuguese passport instead if you can.
Some Seoul-based companies such as Cosmojin Travel and Grace Travel require only a minimum of 24 hours' notice, but their prices are much higher. Also, their itineraries vary, and may not include entry into the DMZ.
The 62-km journey towards Panmunjeom is a sight in itself. The 12-lane Freedom Road becomes eerily empty as you approach the border, as its primary purpose is to get tanks there as fast as possible if war breaks out. To repel an invasion, both sides of the highway, especially the side facing the Imjin River and open water to North Korea, are covered with barbed wire and dotted with observation posts every few hundred meters. Nearby hills house machine gun emplacements, the median strip has clusters of sandbags for defense, and some bridges above the highway have huge concrete blocks that can be dynamited to block the road. Large illuminated signs, proclaiming Freedom and Democracy in hangeul, face the North.
From the North
To visit from the DPRK side is relatively straightforward if it is previously specified as part of your tour (most tours include a day trip to the DMZ from Pyongyang). See North Korea for tour agent listings.
The JSA is 215 kilometers south of Pyongyang. The two are connected by the six-lane Reunification Highway, which much like its Southern counterpart is vast, immaculately maintained and practically empty. Signs along the road count down not the distance to the DMZ, but to Seoul.
From the South, travel within the DMZ is possible only in UN vehicles, and you'll be transferred to a UN bus at Camp Bonifas, accompanied by a member of the U.S. Army who will serve as a tour guide in the JSA.
From the North, you will reboard your tour bus at the entrance to the DMZ, accompanied by a member of the Korean People's Army (KPA).
When booking your tour, be sure to clarify what exactly will be offered. The primary points of interest for most visitors from the south are the Joint Security Area and the Third Tunnel, but not all tour companies have clearance to visit these and you'll have to pay a small premium for those that do.
From the North you will be able to visit the original site of the Panmunjeom village and the Peace Museum that was originally built for and houses original copies of the 1953 armistice. From there it is a short drive to the JSA.
DMZ and JSA
Outside the DMZ
On the South Korean side, sites outside the DMZ can be visited more cheaply and with less hassle. An hourly train runs between Seoul and Imjingak for about ￦ 1300 each way. Bus tours passing the Second & Third Tunnel cost ￦ 8000. This is an excellent alternative for the DMZ tours, though it is not possible to visit Panmunjeom this way.
All tourist facilities for foreigners in the DPRK include a gift shop, and the gateway to the DMZ is no exception. Just inside the concrete wall you can purchase Korean art and amongst other things, of course, endless amounts of literature on the Great Leader Kim Il Sung and his son Kim Jong Il. Prices are reasonable.
"The Monastery" in Camp Bonifas — actually a former officers' club, whose members used to wear brown robes for ceremonies and call themselves the "Merry Mad Monks of the DMZ" — has a gift shop retailing DMZ-related paraphernalia, including chunks of rusty barbed wire from the original demarcation line (W25,000) as well as blueberry wine from North Korea (brought in via a long detour through China).
The canteen in Camp Bonifas is no longer open to the civilian visitors, so most tours now head to eat elsewhere. There is no food available for purchase on the Northern side and the closest available facilities are in Kaesong.
Consumption of alcohol in the DMZ is prohibited and only allowed on special occasions.
There is no accommodation for the general public at Panmunjeom. Most visitors from the south day-trip from Seoul, while visitors from the north either do a long day-trip from Pyongyang or overnight in Kaesong.
Room and board at Camp Bonifas is restricted to active duty soldiers in the United States Armed Forces. However, retired servicemen who have received the Medal Of Honor might be able to stay on a "space available" basis. Contact the United Nations Command Security Battalion - Joint Security Area  for enquiries.
From the South, a strict dress code  applies for all visitors: "faddish, extreme, torn, tattered, frayed, overly provocative or otherwise inappropriate" clothing is not allowed. Sports clothes (incl. tracksuits), military clothing, oversized clothing, sheer clothing, sleeveless shirts/tops, tank tops, anything that bares the midriff or the buttocks, and flip flop-type sandals are specifically banned; clean jeans with a clean T-shirt, on the other hand, are fine. The proclaimed purpose is twofold: one to make sure scruffy hippies don't end up on North Korea's propaganda posters, and the other to make sure ladies in miniskirts and high heels don't trip over and fall if somebody starts shooting.
Within the DMZ, photography outside designated points is not permitted, even from the tour bus. Lenses of up to 90mm focal length are allowed, but tripods are not. (Your camera is not inspected though, so you can probably get away with a longer lens, as long as it's not too obvious.) You must stay together with the group and follow the tour leader's instructions at all times. In the JSA pointing, waving and gesturing are all off-limits. Your group will be asked to walk around in two lines when in sight of the North Korean side.
From the North, visits to the DMZ are more casual than from the South and restrictions are minimal, although it is wise to show a level of respect and etiquette that befits the highly sensitive location. Photography is permitted unrestricted at the JSA and the Peace Museum. Elsewhere you will have to ask permission.
From the South, entry into the JSA/DMZ requires signing a voucher where you agree to accept responsibility for "injury or death as a direct result of enemy action". Remember: it may be in a state of suspended animation, but both sides have itchy trigger fingers and it's still a war zone.
That said, while tensions periodically flare up between North and South, it's been decades (1984, to be precise) since a shot was last fired in anger here. Tour companies can and do suspend tours at short notice if the situation escalates, and it's unlikely you'll be allowed into the DMZ if there's any real risk.
Unless you're planning to risk your life in an attempt to defect, this is not a good place to attempt to cross the border.