  We analyse f(R) modifications of Einstein's gravity as dark energy models in
the light of their connection with chameleon theories. Formulated as
scalar-tensor theories, the f(R) theories imply the existence of a strong
coupling of the scalar field to matter. This would violate all experimental
gravitational tests on deviations from Newton's law. Fortunately, the existence
of a matter dependent mass and a thin shell effect allows one to alleviate
these constraints. The thin shell condition also implies strong restrictions on
the cosmological dynamics of the f(R) theories. As a consequence, we find that
the equation of state of dark energy is constrained to be extremely close to -1
in the recent past. We also examine the potential effects of f(R) theories in
the context of the Eot-wash experiments. We show that the requirement of a thin
shell for the test bodies is not enough to guarantee a null result on
deviations from Newton's law. As long as dark energy accounts for a sizeable
fraction of the total energy density of the Universe, the constraints which we
deduce also forbid any measurable deviation of the dark energy equation of
state from -1. All in all, we find that both cosmological and laboratory tests
imply that f(R) models are almost coincident with a Lambda-CDM model at the
background level.
