This is 2002 (and unchanged for years prior)
(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For dwelling units, conductors, as listed in Table 310.15(B)(6), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service lateral conductors, and feeder conductors that serve as the main power feeder to a dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboard(s)
Of course the NEC means whatever the AHJ says it means but it says this only applies to "the main power feeder ... between the main disconnect and the lighting and appliance panelboard(s)" That would exclude any sub panels