  We consider how well the dark energy equation of state $w$ as a function of
red shift $z$ will be measured using current and anticipated experiments. We
use a procedure which takes fair account of the uncertainties in the functional
dependence of $w$ on $z$, as well as the parameter degeneracies, and avoids the
use of strong prior constraints. We apply the procedure to current data from
WMAP, SDSS, and the supernova searches, and obtain results that are consistent
with other analyses using different combinations of data sets. The effects of
systematic experimental errors and variations in the analysis technique are
discussed. Next, we use the same procedure to forecast the dark energy
constraints achieveable by the end of the decade, assuming 8 years of WMAP data
and realistic projections for ground-based measurements of supernovae and weak
lensing. We find the $2 \sigma$ constraints on the current value of $w$ to be
$\Delta w_0 (2 \sigma) = 0.20$, and on $dw/dz$ (between $z=0$ and $z=1$) to be
$\Delta w_1 (2 \sigma)=0.37$. Finally, we compare these limits to other
projections in the literature. Most show only a modest improvement; others show
a more substantial improvement, but there are serious concerns about
systematics. The remaining uncertainty still allows a significant span of
competing dark energy models. Most likely, new kinds of measurements, or
experiments more sophisticated than those currently planned, are needed to
reveal the true nature of dark energy.
