While the Diablo III launch at times saw essentially all players unable to log in because Blizzard's servers couldn't handle the demand, there were also a number of players reporting that they couldn't log in because their accounts had been stolen. Blizzard today addressed these reports, saying that the security compromises that allowed the account theft in the first place weren't on its end.
In a post on the official Diablo III forums, a Blizzard community manager said, "We've been taking the situation extremely seriously from the start, and have done everything possible to verify how and in what circumstances these compromises are occurring. Despite the claims and theories being made, we have yet to find any situations in which a person's account was not compromised through traditional means of someone else logging into their account through the use of their password."
Blizzard has released a separate statement encouraging all players to consider using the Battle.net Authenticator (a physical keyring that generates access codes) or the Mobile Authenticator app for iOS and Android devices. While the community manager acknowledged the authenticators aren't a guarantee against account theft, "we have yet to investigate a compromise report in which an authenticator was attached beforehand."