  It is shown that, when f'' is non-vanishing, metric f(R) gravity is
completely equivalent to a scalar-tensor theory (with zero Brans-Dicke
parameter) with respect to perturbations of de Sitter space, contrary to
previous expectations. Moreover, the stability conditions of de Sitter space
with respect to homogeneous and inhomogeneous perturbations coincide in most
scalar-tensor theories, as is the case in metric f(R) gravity.
