The notion of temporal uncertainty, expressed as the possibility for an event to occur in an interval of simulation time, rather than at a specific instant, has been recently proposed and exploited in order to enhance the performance of parallel/distributed simulation systems. In this paper we propose the concept of "spatial uncertainty" expressed as the possibility for a simulation event to occur in one of a set of points within the simulated system space. How to handle/exploit space uncertain events in an optimistic simulation system is discussed. Also, experimental results for optimistic simulations of a Personal Communication System (PCS) modeled with space uncertain events are reported. These results show the ability of spatial uncertainty to increase the performance of the parallel simulation system by providing a more flexible approach to synchronization.