Blizzard has warned World of Warcraft players of a Trojan (Disker) that steals Battle.net login information, even if players use an Authenticator.
There are conflicting reports as to whether this particular Trojan is new or not, but it would seem it is not handled by popular antivirus solutions at this point. It's hard to tell how widespread the problem is, but Blizzard are looking into the issue.