The World's best-known Computer Hacker suffered the indignity of having someone break into his new security consulting company's Web site. But Kevin Mitnick shrugged it off as "quite amusing," not serious enough for him to call the FBI. Mitnick, whose federal probation on hacking charges ended a few weeks ago, acknowledged that this weekend's electronic break-in at Defensive Thinking Inc. of Los Angeles was actually the second time in weeks that hackers found a way into the computer running the firm's Web site.