  We review aspects of Cosmic Microwave Background (CMB) spectral distortions
which do not appear to have been fully explored in the literature. In
particular, implications of recent evidences of heating of the intergalactic
medium (IGM) by feedback from active galactic nuclei are investigated. Taking
also into account the IGM heating associated to structure formation, we argue
that values of the y parameter of several*10^(-6), i.e. a factor of a few below
the COBE/FIRAS upper limit, are to be expected. The Compton scattering by the
re-ionized plasma also re-processes primordial distortions, adding a y-type
contribution. Hence no pure Bose-Einstein-like distortions are to be expected.
An assessment of Galactic and extragalactic foregrounds, taking into account
the latest results from the Planck satellite as well as the contributions from
the strong CII and CO lines from star-forming galaxies, demonstrates that a
foreground subtraction accurate enough to fully exploit the PIXIE sensitivity
will be extremely challenging. Motivated by this fact we also discuss methods
to detect spectral distortions not requiring absolute measurements and show
that accurate determinations of the frequency spectrum of the CMB dipole
amplitude may substantially improve over COBE/FIRAS limits on distortion
parameters. Such improvements may be at reach of next generation CMB anisotropy
experiments. The estimated amplitude of the Cosmic Infrared Background (CIB)
dipole might be detectable by careful analyses of Planck maps at the highest
frequencies. Thus Planck might provide interesting constraints on the CIB
intensity, currently known with a ~30% uncertainty.
