The opportunistic networks have shown to be an interesting communication infrastructure for supporting mobile collaborative systems in several application scenarios like disaster relief efforts, hospital work and tourism. The message delivery strategies used in these networks affect in different ways the performance and energy consumption of these systems. Therefore, it is important to understand the message dissemination process in opportunistic networks to envision the impact that it will have on mobile collaborative systems. Most approaches used to study the message dissemination in these networks use simulations or empirical tests. Although useful, the first one requires an important modeling effort, and the second one involves high design and experimentation costs. This limits our capability to evaluate alternatives for message dissemination and choose the most appropriate one according to the features of the application scenario. In order to help address this limitation, we propose a stochastic approach for modeling message dissemination in opportunistic networks. This approach involves a low modeling and usage effort that allows designers not only to compare several dissemination strategies, but also design particular ones using an iterative process. The usability and usefulness of this approach is illustrated through the modeling and analysis of the dissemination process of two well-known message routing strategies. Although the results are still preliminary, they show the feasibility of using this proposal, and opens several opportunities for studying these networks and improving the mobile applications that use them.