For some perspective, Shane Harris reported today that Apple has unlocked phones for the authorities on 70 occasions since 2008. Which is to say, it's possible that TIm Cook's statement is a public relations maneuver in a high-profile case. But, hey - if this motivates Apple to draw the line and take a stand against building tunnels into people's private lives, that's great.
I just saw this: No, Apple Has Not Unlocked 70 iPhones For Law Enforcement
But if they have changed their encryption techniques in recent versions of the iPhone's OS and hardware, then it might not now be possible for them to comply with an order that, in previous versions, may have been feasible on a one-off basis. In other words, perhaps the newer security measures they've introduced make it unfeasible to decrypt the phone without fundamental changes to the OS that would introduce a backdoor to everyone's iPhone, and maybe this wasn't always the case. I don't know whether this is true - can any Apple experts cast light on this?