To me if I trusted AFCI's and they worked as advertised (but that is another thread)
Depending upon how a circuit is wired here is a few ways I would do it:
MWBC:
Put an AFCI receptacle at the first location the MWBC splits up, why worry if we protect some of the existing receptacles?
Single neutral circuit home run to first existing receptacle:
Install AFCI in this first existing receptacle, so what if we protect some of the existing wiring.
Home run to lighting outlet and all receptacles are spidered from L/O to each receptacle, very common in older houses:
This one can present a problem if the circuit is a MWBC, but it seems that code allows that only the new extension has to be protected so in this case just put the AFCI receptacle or blank face AFCI device (when they come out) at the point of the beginning of this extension, or use a two pole to protect both circuits on the MWBC shared neutral.
one problem we might run into is that it was common on older houses to find that a bedroom might have two circuits to it, as it was very common to find that the receptacles on a common wall between two rooms were jumpered from the other room, like a wall between a bedroom and living room would have a circuit that fed receptacles in both rooms on the same circuit.
I think we need to know this and just adapt our installations as we encounter each type of installation, but like was said we need clearer interpretations so we know how we can apply the rule to what we run into.
So far here (in Indiana) we don't have to deal with this as 210.12 was removed from our state codes, and we are jumping from the 2008 to the 2014 NEC and it hasn't been determined yet as to when the 2014 will be put into effect, but I know AFCI requirements will be coming if they can prove to the state AHJ that they do function as the manufactures claim, unlike what has taken place in the past.
