Multimedia-based interfaces to complex services are difficult to create and maintain. An architecture for multimedia presentation systems has been developed in the KIMSAC project, based on a sharp separation between the services and their associated multimedia interfaces. The architecture supports flexibility in designing the multimedia dialogues for individual services, ranging from dialogues that are designed in great detail, to dialogues that are specified only in intentional ways and for which their presentations can be generated or adapted to the context of their use. Specific attention has been given to the needs that characterize open service environments exposed to the public at large. In such environments potentially independent services are accessed in parallel, introducing problems in managing the interleaving of dialogues with several services. The resulting architecture is compared to existing models for UIMS, highlighting the needs to refine these models when applying them to open service environments.