According to Travisa.com as long as you're just transiting airside you don't need a visa. if you wanted to exit the airport you would need a visa unless you have an official or government passport, according to the Korean government's tourism website.
Travisa:
http://korea.travisa.com/TVSVisaInst...R&PartnerID=TA
Visit Korea:
http://www.visitkorea.or.kr/enu/GK/GK_EN_2_1_1.jsp