His boasts were spotted by a Blizzard employee who called the police after seeing that the WoW player had made serious threats. When arrested, the man claimed his computer had been hacked into and then explained he had meant the statements as a joke. He had apparently made them because he didn't believe that, contrary to popular belief, that doing so wouldn't alert the police.
A Blizzard representative is quoted as saying that the chat logs were "very serious" and didn't seem to be just jokes however she declined to show the logs to indystar.com. The man's PC was seized as evidence and the cased turned over to the FBI. We'll let you know more as we hear it but the moral message here is simple: don't threaten to blow up planes in Azeroth. It's not smart.