  The virtual black hole phenomenon, which has been observed previously in
specific models, is established for generic 2D dilaton gravity theories with
scalar matter. The ensuing effective line element can become asymptotically
flat only for two classes of models, among them spherically reduced theories
and the string inspired dilaton black hole (CGHS model).
  We present simple expressions for the lowest order scalar field vertices of
the effective theory which one obtains after integrating out geometry exactly.
Treating the boundary in a natural and simple way asymptotic states, tree-level
vertices and tree-level S-matrix are conformally invariant.
  Examples are provided pinpointing the physical consequences of virtual black
holes on the (CPT-invariant) S-matrix for gravitational scattering of scalar
particles. For minimally coupled scalars the evaluation of the S-matrix in
closed form is straightforward.
  For a class of theories including the CGHS model all tree-graph vertices
vanish, which explains the particular simplicity of that model and at the same
time shows yet another essential difference to the Schwarzschild case.
