The interesting thing in the CA case that's not getting a ton of play is the FBI has the support of the phones legal owner (a government agency) but the owner doesn't know the passcode in this situation.
If the suspected terrorists owned the phone themselves, we'd also have the 5th amendment wrapped up in that case too.
The what-if scenario was if the suspected terrorists owned the phone (as opposed to the deceased's employer). The living neighbor is a suspected terrorist. If he owned the phone and let his now-deceased neighbors borrow the phone, the complications mentioned above could arise.
If the suspected terrorists owned the phone themselves, we'd also have the 5th amendment wrapped up in that case too.