The customer, someone 9to5 Mac calls a "trusted tipster," told the tech Web site that he reached out to Cook after a series of customer support calls to both AT&T and Apple failed to resolve his issue.
The man in question had recently been transferred to Canada for work and decided to try and use his iPhone 3GS (now off-contract, formerly with AT&T) on a Canadian carrier's pay-as-you-go plan, according to 9to5 Mac
Unfortunately for the man, AT&T refused to unlock his iPhone, saying that only Apple was able to do so. Apple, of course, told the man that only AT&T could perform the unlock.
In my personal experience managing an Apple Store and having dealt with a product with two companies sharing responsibilities of customer support, I can say that this situation is very frustrating for consumers. I was often required to bridge the communications gap between AT&T's customer service and our own Genius Bar, untangling the confusing web of who can (and will) do what.
After calling AT&T again, the man was told to jailbreak his iPhone. Jailbreaking your iPhone requires users to perform a number of tasks to help bypass Apple's iOS security implementations, a procedure that on the one hand promises greater flexibility as compared with Apple's "walled garden" approach, but also opens your device to unchecked malware attacks.
And besides, the correct procedure for what this man wanted to do is performing a carrier unlock, which does not involve jailbreaking at all.
In either case, this particular customer preferred Apple's iOS and the security that goes along with it. His last attempt to resolve the issue came through e-mailing Apple CEO Tim Cook. Though Cook did not personally respond to the man's e-mail, later correspondence with AT&T Partnership Operations (confirmed by 9to5 Mac) referenced the man's e-mail to Cook.
The man was then given instructions on how to officially unlock his iPhone through iTunes. Later, Tim Cook's assistant followed up with a phone call to be sure the man's iPhone had been unlocked.
Clearly this is was an isolated incident and most e-mails sent directly to Apple's CEO will probably go unanswered. But, it's nice to see that the CEO of one of the world's top companies still has time to help out a single customer. I think Steve would be proud, don't you?