This excerpt taken from the NDAQ 8-K filed Dec 11, 2006.
Gain on Disposition of Common Stock
A non-U.S. holder generally will not be taxed on gain recognized on a disposition of our common stock unless:
Individual non-U.S. holders who are subject to U.S. federal income tax because the holders were present in the United States for 183 days or more during the year of disposition are taxed on their gains (including gains from the sale of our common stock and net of applicable U.S. losses from sales or exchanges of other capital assets recognized during the year) at a flat rate of 30% or such lower rate as may be specified by an applicable income tax treaty. Other non-U.S. holders subject to U.S. federal income tax with respect to gain recognized on the disposition of our common stock generally will be taxed on any such gain on a net income basis at applicable graduated U.S. federal income tax rates and, in the case of foreign corporations, the branch profits tax discussed above also may apply.