  Dark energy candidates for which the equation-of-state parameter w is less
than -1 violate the dominant energy condition, and are typically unstable. In
scalar-tensor theories of gravity, however, the expansion of the universe can
mimic the behavior of general relativity with w<-1 dark energy, without
violating any energy conditions. We examine whether this possibility is
phenomenologically viable by studying Brans-Dicke models and characterizing
both the naturalness of the models themselves, and additional observational
constraints from limits on the time-dependence of Newton's constant. We find
that only highly contrived models would lead observers to measure w<-1.
