This excerpt taken from the NDAQ 10-K filed Mar 15, 2006.
Delistings may have an adverse effect on our revenues.
Delistings generally increase under poor economic conditions, since issuers are not able to comply with our minimum bid price, market capitalization and/or shareholders equity requirements. Companies are also delisted when they cannot file their periodic reports with the SEC on time. During 2005, 85 companies were delisted for non-compliance with one or more of these requirements and 247 companies voluntarily delisted primarily due to mergers, going private transactions, or changing listing venues. In addition, the SEC has recently proposed new rules which will make it easier for foreign private issuers to delist and stop being U.S. reporting companies. Significant delistings would have a material adverse effect on our financial results.