  In scalar-tensor theories of gravity, the equation of state of dark energy,
w, can become smaller than -1 without violating any energy condition. The value
of w today is tied to the level of deviations from general relativity which, in
turn, is constrained by solar system and pulsars timing experiments. The
conditions on these local constraints for w to be significantly less than -1
are established. It is demonstrated that this requires to consider theories
that differ from the Jordan-Fierz-Brans-Dicke theory and that involve either a
steep coupling function or a steep potential. It is also shown how a robust
measurement of w could probe scalar-tensor theories.
