  We analyse non-equilibrium transport properties of a single-state molecular
quantum dot coupled to a local phonon and contacted by two electrodes. We
derive the effective non-equilibrium (Keldysh) action for the phonon mode and
study the structure of the saddle points, which turn out to be symmetric with
respect to time inversion. Above a critical electron-phonon coupling lambda_c
the effective potential for the phonon mode develops two minima in the
equilibrium and three minima in the case of a finite bias voltage. For strongly
interacting Luttinger liquid leads lambda_c=0. Some implications for transport
experiments on molecular quantum dots are discussed.
