Along with many others, multiple Massively staff received an email (or two) stating that someone had requested a password change, and it definitely wasn't them. In the case of receiving this notification, do as ArenaNet instructs in the email: Nothing. Some folks are also reporting phishing attempts to obtain account information. Never reply to such email, and remember that ArenaNet will never ask for your password.
Players can take steps to increase their account security. Since the log-in name is required to be an email address, use an email dedicated to only the GW2 game account and nothing else. Also, make sure you use a unique (and hard to guess) password and never share it.
[Update: Reader Ring Bonefield sent us this link to a discussion on Reddit in which ANet employees request that those players using an exclusive email address for GW2 file a support ticket to help the studio investigate the claims. ArenaNet has also published on Reddit a list of outstanding issues relating to security, grouping, forums, trading post, and more.]