This excerpt taken from the NDAQ 8-K filed Feb 20, 2008.
Impairment of long-lived assets
Assets with an indeterminable useful life are not depreciated or amortized but tested annually for impairment. Depreciated or amortized assets are assessed for a reduction in value whenever events or changes in conditions indicate that the carrying amount may not be recoverable. The review of the value of all intangible assets takes place on an ongoing basis throughout the year by using a risk-adjusted discounted cash flow. This review is based on assumptions and estimates, such as future cash-flow, useful life and an appropriate discount factor, which entail a certain degree of uncertainty. If the actual outcome deviates from these assumptions and estimates, it will have a material impact on the financial statements.