  While there is mounting evidence in all fronts of experimental cosmology for
a non-vanishing dark energy component in the Universe, we are still far away
from understanding its ultimate nature. A fundamental cosmological constant,
Lambda, is the most natural candidate, but many dynamical mechanisms to
generate an effective Lambda have been devised which postulate the existence of
a peculiar scalar field (so-called quintessence, and generalizations thereof).
These models are essentially ad hoc, but they lead to the attractive
possibility of a time-evolving dark energy with a non-trivial equation of state
(EOS). Most, if not all, future experimental studies on precision cosmology
(e.g. the SNAP and PLANCK projects) address very carefully the determination of
an EOS parametrized ``a la quintessence''. Here we show that by fitting
cosmological data to an EOS of that kind can also be interpreted as a hint of a
fundamental, but time-evolving, cosmological term: Lambda=Lambda(t). We
exemplify this possibility by studying the effective EOS associated to a
renormalization group (RG) model for Lambda. We find that the effective EOS can
correspond to both normal quintessence and phantom dark energy, depending on
the value of a single parameter of the RG model. We conclude that behind a
non-trivial EOS of a purported quintessence or phantom scalar field there can
actually be a running cosmological term of a fundamental quantum field theory.
