Plano is a northern suburb of Dallas. Irving, another Dallas suburb, has several fortune 500 companies, too.
Dallas has one or two, but no company wants to officially move there because of the schools.
Dallas has AT&T, Southwest Airlines, Dean Foods, Energy Futures Holdings (on the last list, now being broken up in bankruptcy, Tenet Healthcare, TI (I thought they were in Richardson, but they show a Dallas address; Metro PCS may be the only Richardson entry), Energy Transfer Equity.
Exxon is the only Irving entry I spotted. That city is more a regional HQ city. Seven Eleven is moving there, but isn’t on the list as it is foreign owned.
Banking is one area where Dallas hasn’t recovered - Comerica is the only major bank headquartered there.