Just announced by the South Korean Embassy in Washington, DC -

As of November 17, 2008, the Republic of Korea joins the U.S. Visa Waiver Program (VWP). As a result, on reciprocity, U.S. citizens can visit Korea without a visa for duration of up to 90 days of less if the purpose of the trip is for pleasure, business, attending a conference, or visiting families or relatives. (Extension of stay is not permitted)

For more information: see South Korea Visa information.