Please review the following definitions in Art 100: Outlet & branch circuit.
A smoke detector is installed at an outlet. A receptacle is installed at an outlet.
[ September 23, 2003, 02:50 PM: Message edited by: tom baker ]
If you are going by the NEC yes smokes are required to be on the AFCI circuit, but in many states and local jurisdictions they have elected to not require it. as here in Indiana the state has reentered the wording of the "99" NEC which said "receptacle outlets" this exempted the lighting circuits as well as the smokes.