  Recent cosmological data have provided evidence for a "dark" relativistic
background at high statistical significance. Parameterized in terms of the
number of relativistic degrees of freedom Neff, however, the current data seems
to indicate a higher value than the one expected in the standard scenario based
on three active neutrinos. This dark radiation component can be characterized
not only by its abundance but also by its clustering properties, as its
effective sound speed and its viscosity parameter. It is therefore crucial to
study the correlations among the dark radiation properties and key cosmological
parameters, as the dark energy equation of state or the running of the scalar
spectral index, with current and future CMB data. We find that dark radiation
with viscosity parameters different from their standard values may be
misinterpreted as an evolving dark energy component or as a running spectral
index in the power spectrum of primordial fluctuations.
