Virgin Mobile USA has filed for an estimated USD506 million initial public offering (IPO). The firm, which is currently owned by Sprint Nextel and the UK’s Virgin Group, has boosted the size of its public sale from the USD100 million that it originally announced. Its filing did not reveal how many shares will be sold or the starting price. Lehman Brothers, Merrill Lynch and Bear Stearns are underwriting the sale. Virgin Mobile USA is a mobile virtual network operator, reselling capacity on the Sprint CDMA network. It launched in 2002 and had 4.9 million customers at the end of March.