Popular gaming news site Eurogamer are reporting today that they have received "multiple reports" of Diablo III accounts being hacked. The story comes complete with the below screenshot, purportedly of a conversation between a genuine user and someone on a compromised account:
According to a user on the Battle.net forums, who claims knowledge from a "hacking" forum, accounts are being compromised by unscrupulous users stealing the "Session ID" of an online game. By doing so, they're able to bypass authentication and other account protection methods (including Blizzard's own Authenticator tool) and gain access to the characters and items of users directly.
A Blizzard Quality Assurance representative responded to the claim, stating "We are very aware of these reports and are taking them very seriously." He also offered advice for anyone that might have been affected, suggesting "If you have been hacked, please contact Customer Service as soon as you can."
Meanwhile, Blizzard have advised that they will be making an official announcement regarding the situation on the general discussion forums soon. In the meantime, it's probably a good idea to avoid playing any public games (or games with people you don't really know) until this situation is resolved.
Update: Blizzard have posted an official response to the claims, stating "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 III."
The remainder of the post goes on to detail the sorts of things you can do to increase the security of your account, including the use of an authenticator and the recently released Battle.net SMS Protect service.