  We consider the consequences of the absence of Birkhoff's theorem in theories
of modified gravity. As an example, we calculate the gravitational force on a
test particle due to a spherical mass shell in the Dvali-Gabadaze-Porrati model
(DGP). We show that unlike in General Relativity, the force depends on the mass
distribution. In particular, the gravitational force within a spherical mass
shell depends on the geometric structure of the bulk, and is likely non-zero.
