Virtual networks are the talk of the wireless world these days. The idea is simple: you provide the brand, and someone else handles the wireless calls behind the scenes. Faced with the imminent loss of AT&T Wireless, AT&T is already considering launching a new virtual network. Even consumer brands like Wal-Mart and Disney have expressed interest in branding their own wireless services. Blame it on the recent success of Virgin Mobile USA -- a partnership between Sprint PCS and Sir Richard Branson's Virgin Group. Since we first wrote about them several months ago, the youth-focused Virgin has continued at its breakneck pace, adding nearly 1 million additional customers (to a total of 1.75 million), making it the nation's fastest growing wireless carrier.