The decoupling of producers and consumers in time and space in the publish/subscribe paradigm lends itself well to the support of mobile users who roam about the environment and have intermittent network connectivity. This paper identifies the factors that affect the performance of a distributed publish/subscribe architecture supporting mobility; formalizes mobility algorithms for distributed publish/subscribe systems and develops and evaluates optimizations that reduce the costs associated with supporting mobility in publish/subscribe systems. In our analysis, we focus on the "unicast" traffic generated to support mobile users, as opposed to the regular "multicast" traffic used for event dissemination to stationary clients. We find that the network capacity must be doubled to handle the extra load of just 10% of mobile users.