  In order to obtain information about the Standard Model from exclusive
hadronic two-body B-decays, we have to quantify non-perturbative QCD effects.
Approaches based on the factorization of mass singularities into hadronic
distribution amplitudes and form factors provide a rigorous theoretical
framework for the evaluation of these effects in the heavy quark limit. But it
is not possible to calculate power corrections in a model-independent way,
because of non-factorizing long-distance contributions. It has been argued that
Sudakov effects suppress these contributions and render the corresponding
corrections perturbatively calculable. In this paper we examine this claim for
the related example of semileptonic B -> pi decays and conclude that it is not
justified. The uncertainties in our knowledge of the mesons' distribution
amplitudes imply that the calculations of the form factors are not sufficiently
precise to be useful phenomenologically. Moreover, a significant contribution
comes from the non-perturbative region of large impact parameters, and is
therefore uncalculable. We also raise a number of theoretical issues in the
derivation of the underlying formalism. For B-decays it is thus not possible to
invoke Sudakov effects to calculate amplitudes for decays which have
long-distance divergences (end-point singularities) in the standard
hard-scattering approach.
