  We have reanalysed constraints on the equation of state parameter, w_Q =
P/rho, of the dark energy, using several cosmological data sets and relaxing
the usual constraint w_Q > -1. We find that combining Cosmic Microwave
Background, large scale structure and Type Ia supernova data yields a
non-trivial lower bound on w_Q. At 95.4% confidence we find, assuming a flat
geometry of the universe, a bound of -2.68 < w_Q < -0.78 if w_Q is taken to be
a completely free parameter. Reassuringly we also find that the constraint w_Q
> -1 does not significantly bias the overall allowed region for w_Q. When this
constraint is imposed the 95.4% confidence bound is -1 < w_Q < -0.71. Also, a
pure cosmological constant (w = -1) is an excellent fit to all available data.
Based on simulations of future data from the Planck CMB experiment and the SNAP
and SNfactory supernova experiments we estimate that it will be possible to
constrain w_Q at the 5% level in the future.
