Thanks to all,
Now I was playing around with Phoenix on my binding problem here as well (which is part of the reason for my confusion)... am I right to think that phoenix has a "result" template incompatible with result_of?

The following seems to work when using Phoenix lazy functions, etc., but the standard result template did not.
struct test_function_impl
{
    //For phoenix... not sure if necessary for
   template<typename Arg1, typename Arg2>
   struct result {
        //typedef typename Arg1::value_type type;
   };

    template<typename Vector, typename IntT>
    typename Vector::value_type //The type in the vector... can be a double or AD type
    operator()(const Vector& x, IntT y) const{
        return x[0] * x[0] + y;
    }
};

Thanks,
Jesse