  We reconsider the issue of superluminal propagation in the DGP model of
infrared modified gravity. Superluminality was argued to exist in certain
otherwise physical backgrounds by using a particular, physically relevant
scaling limit of the theory. In this paper, we exhibit explicit
five-dimensional solutions of the full theory that are stable against small
fluctuations and that indeed support superluminal excitations. The scaling
limit is neither needed nor invoked in deriving the solutions or in the
analysis of its small fluctuations. To be certain that the superluminality
found here is physical, we analyze the retarded Green's function of the scalar
excitations, finding that it is causal and stable, but has support on a widened
light-cone. We propose to use absence of superluminal propagation as a method
to constrain the parameters of the DGP model. As a first application of the
method, we find that whenever the 4D energy density is a pure cosmological
constant and a hierarchy of scales exists between the 4D and 5D Planck masses,
superluminal propagation unavoidably occurs.
