Front propagation in two dimensional steady and unsteady cellular flows is investigated in the limit of very fast reaction and sharp front, i.e. in the geometrical optics limit. In the steady case, by means of a simplified model, we provide an analytical approximation for the front speed, $v_f$, as a function of the stirring intensity, U, in good agreement with the numerical results. The main contribution to $v_f$, comes from the large scale dynamics and, for sufficiently high U-values, $v_f \sim U/\ln U$ closely resembling the behavior proposed for turbulent flows. In the unsteady (time-periodic) case, the front speed displays a phase-locking on the flow frequency and, albeit the Lagrangian dynamics is chaotic, chaos in front dynamics only survives for a transient. Asymptotically the front dynamics is periodic and chaos manifests only in the spatially wrinkled structure of the front.