It is now commonplace for a person to use lightweight wireless computing devices, and to make his/her data available to other people's devices using todays various networking capabilities (infrastructurebased WLAN, ad hoc WLAN, GSM, etc.). Middleware platforms initially developed for stationary distributed systems cannot be directly applied in such a mobile environment. They must adapt their functionalities so as to best cope with possible resource constraints (energy, storage) of mobile terminals as well as with the various types of wireless networks that are now available. In this paper, we present a middleware service that allows collaborative data sharing among ad hoc groups that are dynamically formed according to the connectivity achieved by the ad hoc WLAN. Our service enhances, in particular, data availability within mobile ad hoc collaborative groups, and integrates a new adaptive data replication protocol for mobile terminals, combining both optimistic and conservative schemes. Our service has been designed so as to minimize energy consumption and optimize data availability and storage consumption.