We have reanalysed constraints on the equation of state parameter , w _ { Q } \equiv P / \rho , of the dark energy , using several cosmological data sets and relaxing the usual constraint w _ { Q } \geq - 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 } \geq - 1 does not significantly bias the overall allowed region for w _ { Q } .
When this constraint is imposed the 95.4 % confidence bound is -1 \leq 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 .