Not necessarily. Don't you sign the contract at activation, and not when you buy the phone? If that's the case, you could buy an iPhone and then hack it to another provider. That's not breaking a contract if you never signed one.
And therein lies the rub! You, would not necessarily be breaking a contract with AT&T, but wouldn't Apple?
Apple would be breaking the contract it has with at&t regarding exclusivity of the iPhone if hackers find workarounds either through Apple's hardware component or software part. It's up to Apple to maintain the legitimacy of it's agreement with at&t by updating the iPhone to keep hackers from causing apple to break their contract with at&t. Not to mention, don't expect new iPhone apps if apple software team has to keep having their attention diverted to hackers who think they are in the right with regards to unlocking the phone.