[csandreasen]

By that same logic, Apple's source code and software update key could eventually leak, too, which would have even worse effect. The FBI is handing the phone over to Apple to apply the modification, not asking Apple to them the means to do it themselves. If Apple was really worried about this leaking, they could just delete it when they're done (though it would be more work for them to recreate it next time the FBI comes with a warrant to search another iPhone).