This excerpt taken from the NDAQ 8-K filed Feb 20, 2008.
Cost of acquisition
Under IFRS, equity shares issued as purchase consideration in a business combination are valued based on the weighted average trading price of the shares shortly before and after the date of the exchange transaction, which is the date when the acquirer obtains control over the acquirees net assets and operations. Under U.S. GAAP, such equity shares are valued based on the average share price for a reasonable period before and after the acquisition is publicly announced or, if the number of shares is uncertain on such date, the first day on which both the number of acquirer shares and the amount of other consideration becomes fixed. OMX has identified a difference between U.S. GAAP and IFRS relating to share value at OMXs acquisitions of the Stockholm, Helsinki, Copenhagen and Iceland stock exchanges in the amount of $18.1 million, $(45.9) million, $2.8 million and $3.0 million, respectively. Such differences affect the amount of goodwill recorded.