It's time to cough up the cash, Bill Gates and Warren Buffett are telling their fellow billionaires. A Fortune magazine story this month describes the superwealthy pair’s plan to ask the 400 wealthiest Americans to pledge half of their net worth to charity. The two got started in May 2009 with a top-secret dinner in New York City which featured David Rockefeller, Michael Bloomberg, George Soros, Oprah Winfrey, and Ted Turner. Melinda Gates told the magazine that getting both husbands and wives on board will be key to their success. "Even if he's the one that made the money, she's going to be a real gatekeeper," Melinda Gates said. "And she's got to go along with any philanthropic plan, because it affects her and it affects their kids."