  Dewetting of thin polystyrene films deposited onto silicone wafers at
temperatures close to the glass transition exhibits unusual dynamics and front
morphologies. Here, we present a new theoretical approach of these phenomena
taking into account both the viscoelastic properties of the film and the
non-zero velocity of the film at the interface with the substrate (due to
slippage). We then show how these two ingredients lead to : (a) A very
asymmetric shape of the rim as the film dewetts, (b) A decrease of the
dewetting velocity with time like $t^{-{1/2}}$ for times shorter than the
reptation time (for larger times, the dewetting velocity reaches a constant
value). Very recent experiments by Damman, Baudelet and Reiter [Phys. Rev.
Lett. {\bf 91}, 216101 (2003)] present, however, a much faster decrease of the
dewetting velocity. We then show how this striking result can be explained by
the presence of residual stresses in the film.
