Re: arc faults
gimpy,
what jim is saying is the NEC does not require the installation of the smoke detectors.
If your local authority requires them or if you are installing them on your own accord (I would reccommend some type of smoke detector system) then the smoke detector system would have to meet the applicable articles, ie. AFCI protection, box fill, wire support/securing, etc....
As Don stated, Code is not retroactive, but you can choose to install extra AFCI breakers.
Bear in mind there can be local ammendments that you would have to follow.