Minecraft users should change their passwords and those of any accounts using the same log-in details.
Minecraft’s user data servers have apparently been compromised, with user names and passwords leaked online in plain text. 1,800 Mojang accounts were affected.
Mojang later confirmed that user info has been leaked, but said there was no hack – the details were obtained through other means, like phishing.
In any case, it’s best to just go change your password; it doesn’t hurt to do it regularly anyway.