Americans have been signing up for high-speed service in big numbers. The major cable and phone companies added more than 2 million new subscribers in the third quarter, raising the total number of broadband households to more than 22 million. Meanwhile, The Yankee Group estimates there will be 51.5 million dial-up households at the end of December, down from 54.5 million a year ago. Denice Hasty, assistant vice president of consumer marketing at San Antonio-based SBC, said about 70% of DSL subscribers have moved up from slow dial-up service.