I will also add my suspicion that the FBI already knows what's on the phone, or simply doesn't care. Supposedly, in some goof-up during the investigation, the FBI botched a password reset attempt to gain access to the iCloud account linked to the phone, so that the phone couldn't back up its precious contents to the cloud--where Apple apparently can help extract them. Oops, so sorry, here comes the All Writs Warrant for Apple to create the firmware bypass to the 10-and-out function on the phone itself.
If I was Apple (and the Obama administration and, for that matter, people who worry about PRC bullying of US IT firms for access to source code, surveillance utilities and the like) I would look for a graceful way to cave in response to a one-time demand through a court in a single case. Better to button up this issue now, in other words, rather than open the door for the Congress to pass a CALEA-style law with a blanket obligation for Apple to cooperate on issues of this sort--a precedent that would make the PRC pretty happy.
Updated on Feb. 23, 2016 with some additional observations on the San Bernardino phone and the court order vs. legislation angle.