We studied the dewetting process of thin polystyrene (PS) films on silicon substrates, coated with a thin, irreversibly adsorbed polydimethylsiloxane (PDMS) layer, by optical microscopy and atomic force microscopy. Besides demonstrating the exceptional potential of dewetting for a sensitive characterization of rheological properties of PS thin films, characterized by a stress-relaxation time, tau(1), we focused on the influence of the frictional behaviour (energy dissipation mechanism) at the interface between the PDMS-coated silicon wafer and the PS film on the dewetting process. Our results show that the initial stages of dewetting depend sensitively on the thickness and the way the PDMS layer was adsorbed. The maximum width of the dewetting rim at tau(1) increased with increasing PDMS layer thickness, which can be interpreted as an increase of the effective, velocity-dependent slippage length. Interestingly, tau(1) was found to be almost independent of the substrate properties. Our results demonstrate that dewetting is a really powerful approach for rheological and frictional studies of thin polymer films.