You can fly to Seoul from Vladivostock, Ulan-Bator, Beijing and a bunch of other Chinese cities. In addtion, if you are really a glutton for punishment, you can take the train from Beijing to Tianjin and then take the ferry to Inchon. From Inchon you can take the subway to Seoul. The Lonely Planet guide to China has lots of info about taking the ferry.
Paul Carver