[January 2011 update: This information is a bit outdated. For more up-to-date information on getting a Korean driver's license, visit "Getting a Korean Driver's License".]
This just in from the American Embassy:
Korean authorities have told us that effective February 2010, the Korean government will require American citizens to bring a notarized affidavit from the U.S. Embassy attesting to the authenticity of your U.S.-issued driver’s license in order to obtain a Korean license. You may come in anytime during our office hours to execute this affidavit. The notarial fee is $30.00 ($20.00 for each subsequent notarial). Please visit our website for our office hours and more details: http://seoul.usembassy.gov/
I'm not sure if this means you still have to take the computer test or if the affidavit and a valid US license qualifies you for a K license automatically.