  We demonstrate that cosmic microwave background observations consistent with
a cosmological constant universe predict in a well-defined sense that lower
redshift measures will nearly automatically deliver w=-1 for the dark energy
equation of state value unless they are sensitive to w(z). Thus low redshift
data pointing to w=-1 does not truly argue for a cosmological constant. Even
the simplest question of whether the equation of state of dark energy is equal
to the cosmological constant therefore requires experiments able to sensitively
constrain time variation w(z) and not merely a constant w. We also note a
number of issues regarding parametrization of w(z), demonstrating that the
standard form w(z)=w_0+w_a z/(1+z) is robust but use of high order polynomials
and cutting off the high redshift behavior can be pathological.
