We recently analyzed the dynamics of wetting for polymer melts on smooth solid surfaces with a few grafted chains (nu chains per unit area). For nu = 0 ("ideal" surface), we expect a strong slippage of the interface. For small nu("semi-ideal" surface), recent theoretical and experimental studies reveal a transition between a low-velocity, nonslipping regime and a high-velocity, slipping regime, where the shear stress takes a fixed value, sigma*. We investigate here the consequences of this transition on wetting and dewetting processes. Our discussion concentrates first on partial wetting conditions, which give relatively large flow velocities, and thus allow the slip regime to be reached. For dewetting processes, we find that a dry patch should first grow with a radius of R(t) almost-equal-to t2/3 in a strong slippage regime, and then shift to a nonslip regime (R almost-equal-to t) when the size R exceeds a certain critical value, R(c). A t2/3 law has indeed been observed in experiments by C.R. The limiting radius R(c) depends strongly on the grafting density and on the contact angle, but may be typically in the millimeter range. We also discuss the case of complete wetting : here slippage is expected to be important only for large dynamic contact angles theta(d); below a critical dynamical angle, theta(c), slippages should be suppressed and the shape of the spreading droplet should simply be a spherical cap. Above theta(c), we expect a spherical cap plus a protruding (macroscopic) "foot", and the precursor film becomes independent of the velocity.