  Models of dark energy are conveniently characterized by the equation-of-state
parameter w=p/\rho, where \rho is the energy density and p is the pressure.
Imposing the Dominant Energy Condition, which guarantees stability of the
theory, implies that w \geq -1. Nevertheless, it is conceivable that a
well-defined model could (perhaps temporarily) have w<-1, and indeed such
models have been proposed. We study the stability of dynamical models
exhibiting w<-1 by virtue of a negative kinetic term. Although naively
unstable, we explore the possibility that these models might be
phenomenologically viable if thought of as effective field theories valid only
up to a certain momentum cutoff. Under our most optimistic assumptions, we
argue that the instability timescale can be greater than the age of the
universe, but only if the cutoff is at or below 100 MeV. We conclude that it is
difficult, although not necessarily impossible, to construct viable models of
dark energy with w<-1; observers should keep an open mind, but the burden is on
theorists to demonstrate that any proposed new models are not ruled out by
rapid vacuum decay.
