The United States has never experienced <A HREF="http://news.zdnet.com/2100-9589_22-6087470.html?tag=zdnn.alert">a massive Internet outage,</A> but a coalition of dynamic chief executives said Friday that the nation must do more to prepare for that prospect. The cautionary document was a product of the Business Roundtable, whose 160 corporate members include companies ranging from Hewlett-Packard, IBM and Sun Microsystems to General Motors, Home Depot and Coca-Cola. All told, the group's high-rolling membership counts $4.5 trillion in annual revenues, more than 10 million employees, and nearly a third the total value of the U.S. stock market.