  The time-dependent CP asymmetry in exclusive B^0(t) -> K^{*0}\gamma decays
has been proposed as a probe of new physics in B decays. Recently, this method
was extended to radiative decays into multibody hadronic final states such as
B^0(t)-> K_S\pi^0\gamma and B^0(t)-> \pi^+\pi^-\gamma. The CP asymmetry in
these decays vanishes to the extent that the photon is completely polarized. In
the Standard Model, the photon emitted in b-> s\gamma has high left-handed
polarization, but right-handed contamination enters already at leading order in
\Lambda/m_b even for vanishing light quark masses. We compute here the
magnitude of this effect and the time dependent CP asymmetry parameter S_{K_S
\pi^0\gamma}. We find that the Standard Model can easily accomodate values of S
as large as 10%, but a precise value cannot be obtained at present because of
strong interactions uncertainties.
