Reports of wide spread hacking of Battle.net accounts over the weekend prompted Joystiq to contact Blizzard for an official comment:
"Historically, the release of a new game -- such as a World of Warcraft expansion -- will result in an increase in reports of individual account compromises, and that's exactly what we're seeing now with Diablo 3," Blizzards told Joystiq this evening. "We know how frustrating it can be to become the victim of account theft, and as always, we're dedicated to doing everything we can to help our players keep their Battle.net accounts safe -- and we appreciate everyone who's doing their part to help protect their accounts as well."
Keep it hidden, keep it safe. Or use an authenticator in other words.