The new rustbelt? Rearden Steel, Inc. is building its new factory in the heart of San Francisco's "Multimedia Gulch." But don't think the neighborhood is stepping back from the digital age. The only steel elements in the unfinished loft are the fashionable blue-steel design structures and the catwalks that overlook the tangle of wires and piles of computers. Rearden Steel is a cover for the new company being formed by Steve Perlman, the co-founder and recently departed president of Microsoft Corp.'s WebTV Networks unit. Mr. Perlman provided some clues to what he is up to at WebTV's holiday party in January, where a professional dancer strapped on motion sensors and controlled a 3-D character dancing on a screen. Another hint: The name comes from the factory owned by a character in Ayn Rand's 1957 classic, "Atlas Shrugged." Readers of the novel discover, among other things, "why a productive genius becomes a worthless playboy," according to the publisher. Mr. Perlman points out the genius is really masquerading. He's secretive about the commercial possibilities of his latest tinkerings, but, in the time-honored tradition of Silicon Valley, he claims, "If it works, it just might change everything."