  I revisit the question of the adiabaticity of initial conditions for
cosmological perturbations in view of the 3-year WMAP data. I focus on the
simplest alternative to pure adiabatic conditions, namely a superposition of
the adiabatic mode and one of the 3 possible isocurvature modes, with the same
spectral index as the adiabatic component.
  I discuss findings in terms of posterior bounds on the isocurvature fraction
and Bayesian model selection. The Bayes factor (models likelihood ratio) and
the effective Bayesian complexity are computed for several prior ranges for the
isocurvature content. I find that the CDM isocurvature fraction is now
constrained to be less than about 10%, while the fraction in either the
neutrino entropy or velocity mode is below about 20%. Model comparison strongly
disfavours mixed models that allow for isocurvature fractions larger than
unity, while current data do not allow to distinguish between a purely
adiabatic model and models with a moderate (ie, below about 10%) isocurvature
contribution.
  The conclusion is that purely adiabatic conditions are strongly favoured from
a model selection perspective. This is expected to apply in even stronger terms
to more complicated superpositions of isocurvature contributions.
