6 Hours in Seoul

Allow 90 mins to get into the city, and another 90 to get back to Incheon (depending pn time of day).

I say no, too risky. You need 30 mins to clear customs, conservatively 3 hours to travel to and from the airport, and 90 mins prior to check in, leaves you with 1 hour to see Seoul. I travel to Seoul regularly for work, we land at 10pm and have a hotel car pick us up (no no waiting time) and I have never got downtown in less than 110 mins from the plane landing. Incheon airport is meant to be the best in the world, relax in the spa or enjoy some Korean food before your long flight to London.

Asiana offers a free transit tour, but you may need to have an 8 hour layover. Worth checking out though.

6 hours--I understand your desire to see a bit of Seoul, but it is not worth the risk with respect to missing a flight. My advice: stay at the airport.

How about a seven hour layover?

I arrive in Seoul 12.25 on Asiana from Frankfurt, my onwards flight to Hanoi departs 7.35pm, so I have seven hours, which I do NOT want to spend in the airport all the time (I kinda booked Asiana on purpose because I wanted to seize the chance to take a quick look at Seoul/Korea since I've already seen the other convenient layover points like Kuala Lumpur for Malaysia Airlines or Singapore). I found the 5-hour-tour the Korea Tourism Board offers but I find it rather expensive and perhaps a bit too long - if I can't board this tour straight after clearing customs, I won't make it for my onwards flight. So I thought of just taking whatever public transport there is into Seoul, wander around for 1-2 hours and go back.

So how doable this is? If I can only pick 1-2 spots in Seoul, what's the ultimate must-see?

Since I will fly on to Hanoi I guess I don't need to pick up my luggage or anything, I can just walk through customs with my cabin luggage and come back in the evening?

Sorry for asking stupid questions, but I've actually never done this before, so far I arranged my layovers so that I'd actually take my baggage, stay overnight, etc. before going onwards.

As before, it'll take you between 60-90 mins to get into Seoul. Your best bet is the Airport Limousine, an express bus service that runs to various parts of the city. The ticket booths are located in arrivals. Ask at Information if you can't find them.
Jongno would be a sensible place to head for, as there are a few palaces and temples round there, as well as giving you a good taste of the modern city too. If you are heading out of Seoul late afternoon/early evening, I'd allow the maximum time to get back to Incheon again, as traffic will be heavy. Check that your baggage is cleared all the way to Hanoi when you check it in at Frankfurt, but I'd be very surprised if it isn't.

An easier (although less interesting) option, would be to forget about Seoul, and just go into Incheon city instead. You'll get just as much of a flavour of Korea, and more time to wander around.

Seoul Eats has some delicious looking suggestions if you want to stay in the Incheon area: http://www.seouleats.com/2009/08/have-layover-in-incheon-airport-what.html

Thanks for the advice so far. I've done a bit of "homework" and decided to try and see Changdeokgung and Gyeongbokgung Palaces and Jogyesa Temple, since these are all pretty close to each other (Is that the "Jongno" area?). I can go without food if necessary, since Asiana will hopefully feed me between Frankfurt and Seoul and later between Seoul and Hanoi.

So I assume that I should take a bus to somewhere in the city where I can change to the subway. Should I try for Seoul East/West/South or North? I have a subway map but I have no idea where the bus stops are... :(

If traffic gets busy in the late afternoon (of course it would be!), should I perhaps try to go back on the subway instead of the bus? I guess I could take subways to Gimpo Airport and then take the train to Incheon?

Ooo, I hope AREX gets the train extension to Seoul Central Station going in time, it says on the website "to be opened in 2010" and the journey will take 40mins. It would be the answer to all my problems.

You want North. If you take the bus that goes to the hotels, try to go to the Lotte Hotel, or Koreana, they are sort of close to where you want to go.