  We propose the novel mechanism leading to superfluidity breakdown in dilute
Bose-condensed gases. We discuss the properties and highlight the role played
by surface excitations in trapped condensates. We show that the critical
velocity measured in recent experiments is nothing else but the critical
velocity associated with the spontaneous creation of surface modes. The latter
turns out to be larger than that found by the analysis of a superflow stability
with respect to vortex nucleation.
