  We discuss the potentially important role played by preheating in certain
variants of the curvaton mechanism in which isocurvature perturbations of a
D-flat (and F-flat) direction become converted to curvature perturbations
during reheating. We analyse the transition from inflation to reheating in some
detail, including the dynamics of the coupled curvaton and inflation fields
during this transition. We discover that preheating could be an important
source of adiabaticity where parametric resonance of the isocurvature
components amplifies the super-horizon fluctuations by a significant amount. As
an example of these effects we develop a particle physics motivated model which
we recently introduced in which the D-flat direction is identified with the
usual Higgs field. Our new results show that it is possible to achieve the
correct curvature perturbations for initial values of the curvaton fields of
order the weak scale. In this model we show that the prediction for the
spectral index of the final curvature perturbation only depends on the mass of
the curvaton during inflation, where consistency with current observational
data requires the ratio of this mass to the Hubble constant to be < 0.3.
