In order to improve the usability of the Square Enix Account Management System, customers will be able to login to the Square Enix Account Management System using their registered email address starting on Jan. 9, 2013 at 8:00 p.m. (PST).
Customers will be able to login using their Square Enix ID / password or registered email address / password once the changes are in effect.
Please note that these login changes will only apply to customers that are logging into the Square Enix Account Management System. Customers attempting to login to games and official websites using their Square Enix account will be required to login using their Square Enix ID / password as usual.
Square Enix Account Management System
https://secure.square-enix.com/account/