For a continuous wall footing, in the direction of the wall, ACI 318-02 22.7 allows for unreinforced footings. The reason is - that it is OK for such a footing to crack. It will still provide it's structural function, which is to spread the load to the soil. And no one will see a crack, as it is buried.
in addition to the ACI 318 requirements, some local jurisdictions have a minimum requirement of 2 #5 in the bottom of a strip footing and #5 at 12" o.c. each way in the bottom of a spread footing. Even if the engineering shows no rebar is required.