This excerpt taken from the AMZN 10-Q filed Oct 22, 2008.
Goodwill and Long-Lived Assets
Goodwill is tested for impairment annually and when an event occurs or circumstances change such that it is reasonably possible that an impairment may exist. Our annual testing date is October 1. We test goodwill for impairment by first comparing the book value of net assets to the fair value of the related operations. If the fair value is determined to be less than book value, a second step is performed to compute the amount of impairment. In this process, a fair value for goodwill is estimated, based in part on the fair value of the operations, and is compared to its carrying value. The shortfall of the fair value below carrying value represents the amount of goodwill impairment.