This excerpt taken from the NDAQ 8-K filed Feb 20, 2008.
According to the U.S. accounting standard SFAS 142, Goodwill and Other Intangible Assets, applicable from January 2002, acquisition goodwill and other intangible assets that have indefinite useful lives are not amortized, but are instead tested for impairment annually. IFRS similarly requires annual testing of impairment. Prior to the implementation of IFRS in January 2004, OMX, under Swedish GAAP, amortized goodwill and other intangible assets over their expected useful lives. As a result, differences between U.S. GAAP and IFRS arise from the different dates of implementation.