After years of loudly challenging the theory of global warming, a growing number of energy companies are saying the regulation of greenhouse gases in the U.S. is inevitable and are even lining up behind proposed legislation to do so. Five of the country's largest electric power companies, including Exelon, Entergy and Calpine, said Wednesday they would back a bill sponsored by Democrats to create a so-called "cap and trade" system in the U.S. ...And at a carbon conference in Washington, D.C., officials with...BP and Royal Dutch Shell said the U.S. must develop a regulatory system that ties in with one now being developed...under the Kyoto Protocol. Even Exxon Mobil, one of the most vocal skeptics of global warming, has softened its stance.