Sprint (NYSE: S) is facing a shareholder revolt, with activist investor Ralph Whitworth “threaten[ing] a proxy fight for board seats unless Sprint directors “immediately” deal with the company’s leadership”. Whitworth’s Relational Investors LLC owns about 53.1 million shares, or roughly 1.9 percent of outstanding Sprint shares, according to the WSJ, and he has a number of CEO scalps on his belt. “We have lost confidence in [Sprint CEO] Gary Forsee,” Mr. Whitworth said in an interview yesterday. Since taking a stake in Sprint earlier this year, Mr. Whitworth has raised concerns with management about the rationale for its investment in a new but relatively untested wireless-broadband technology, WiMax, as well as what he deems lackluster attention to its core cellphone business. He also asked the company to consider the potential sale of its fiber-optic-networking and long-distance operations.” The big concern appears to be the lackluster performance of Sprint’s mobile business, which has added fewer customers than rival AT&T (NYSE: T), and doubt that the roll-out of the WiMax will bring enough return to cover the costs. Sprint has said the network will cost $5 billion to roll out by 2010, and bring in $2-2.5 billion in annual revenue by that time.