This is a difficult city for finding places. Having a map is essential, but if you need to go to a specific address that is not a major hotel or a landmark building or site, you will need a map.
It is best to get the place you are visiting to fax you a map to give to the taxi driver or for arranging your transport on the buses or metro.
It's hard to find a good street map of Seoul when you're not in Seoul at the moment. This interactive map is pretty good and printable. Has the metro lines and names some of the building and sights to see.
Your hotel should have several great maps to provide you.
I was in Seoul about three years ago. Then the street names were solely in Korean. I was sure that getting lost in Seoul would be a piece of cake if I were travelling on my own.
My return to Seoul this time, I am very pleased and suprised to see that english and sometimes chinese names were added to the street signs. This is much more assuring for travellers who are not familiar with the korean language.
They are still in the midst of updating the street names. At least till 2007 would most of it be completed.