"In May 2003 alone, Linux-based corporate and government systems experienced 19,208 successful breaches worldwide, whereas similarly oriented Microsoft Windows Server systems suffered only 3801 breaches. During this time period, more than 75 percent of all server-based breaches occurred on Linux systems; Windows systems were responsible for just 15 percent of breaches," says WinInformant. There's a press release on the mi2g site which provided the numbers, but you have to pay $100 to read it.