We propose a novel distributed random algorithm for self-deployment of a network of mobile wireless sensors in the problem of blanket coverage. The aim is to deploy sensors in a bounded region so that any point of the region is sensed by at least one sensor. The algorithm is based only on information about the closest neighbours of each sensor. The monitoring region is of an arbitrary shape and not known to the sensors a priori. We give mathematically rigorous proofs of asymptotic optimality and convergence with probability 1 of the proposed algorithm.