Using Monte Carlo simulations, we study the mobility of a harmonic triangular lattice subjected to an incommensurate short-wavelength potential and a steady driving force. At zero temperature (T = 0) the mobility jumps from zero to a finite value at a critical force . For T>0 the mobility shows Arrhenius behaviour, and grows exponentially with driving force before saturating at the free-particle limit for . We find no evidence of a sharp depinning transition at finite temperature. This suggests that observations of depinning of charge-density waves, and the irreversibility line in high-temperature superconductors, may simply represent the onset of detectable motion on laboratory timescales, rather than an underlying phase transition.