  We calculate the dominant Standard Model contributions to the time-dependent
CP asymmetry in B0->K*0 gamma, which is O(1/mb) in QCD factorisation. We find
that, including all relevant hadronic effects, in particular from soft gluons,
the asymmetry S is very small, S=-0.022\pm 0.015^{+0}_{-0.01}, and smaller than
suggested recently from dimensional arguments in a 1/mb expansion. Our result
implies that any significant deviation of the asymmetry from zero, and in
particular a confirmation of the current experimental central value,
S_{HFAG}=-0.28\pm 0.26, would constitute a clean signal for new physics.
