  In several classes of particle physics models -- ranging from the classical
Majoron models, to the more recent scenarios of late neutrino masses or
Mass-Varying Neutrinos -- one or more of the neutrinos are postulated to couple
to a new light scalar field. As a result of this coupling, neutrinos in the
early universe instead of streaming freely could form a self-coupled fluid,
with potentially observable signatures in the Cosmic Microwave Background and
the large scale structure of the universe. We re-examine the constraints on
this scenario from the presently available cosmological data and investigate
the sensitivity expected from the Planck satellite. In the first case, we find
that the sensitivity strongly depends on which piece of data is used. The SDSS
Main sample data, combined with WMAP and other data, disfavors the scenario of
three coupled neutrinos at about the 3.5$\sigma$ confidence level, but also
favors a high number of freely streaming neutrinos, with the best fit at 5.2.
If the matter power spectrum is instead taken from the SDSS Large Red Galaxy
sample, best fit point has 2.5 freely streaming neutrinos, but the scenario
with three coupled neutrinos becomes allowed at $2\sigma$. In contrast, Planck
alone will exclude even a single self-coupled neutrino at the $4.2\sigma$
confidence level, and will determine the total radiation at CMB epoch to
$\Delta N_\nu^{eff} = ^{+0.5}_{-0.3}$ ($1\sigma$ errors). We investigate the
robustness of this result with respect to the details of Planck's detector.
This sensitivity to neutrino free-streaming implies that Planck will be capable
of probing a large region of the Mass-Varying Neutrino parameter space. Planck
may also be sensitive to a scale of neutrino mass generation as high as 1 TeV.
