If the iPhone of the San Bernadino shooters is locked, how would it even be possible for Apple to acquiesce to the FBI's request for them to unlock the phone? Does that mean Apple has some kind of "back door" that would let them into any phone? Wouldn't a locked phone theoretically be secured from access by anyone who doesn't have the password, including the manufacturer?
Apple would have to develop a version of iOS that bypasses delays in passcode entries and disables the setting which deletes the phone's data after the 10th failed attempt. The phone would then be brute forced. FBI would also like to enter passcode through a connection to the phone rather than the current UI.
Apple would then most likely load this onto the phone using a tool referred to as PurpleRestore. This is the best guess as to how they would load a new version of iOS without unlocking the phone.