It hasn't been a great day for AT&T. Earlier this morning, its data network went down in the northeast, and now customers are receiving emails informing them of a potential security breach in which hackers attempted to gain access to their online accounts. However, it appears there's no cause for alarm — AT&T claims that it managed to shut down the attack before any sensitive information was transmitted, and that less than one percent of customers were potentially affected. In a year where Sony and Citigroup have both suffered crippling online security attacks, this has to be considered a success for AT&T. Perhaps it's learned from last year's iPad security breach in which over 100,000 iPad users had their email addresses exposed to hackers. Still, if you're an AT&T customer, today is probably a good time to change your password.