This paper describes a novel model for the service selection problem of workflow-based applications in the context of self-managing situated computing. In such systems, the execution environment includes different types of devices, from remote servers to personal notebooks, smartphones, and wireless sensors, which build an infrastructure that can dynamically change both its physical and logical architecture at run-time. We assume that worflows are defined abstractly; i.e., they invoke abstract services whose concrete counterparts can be selected dynamically. We also assume that concrete service implementations may possibly migrate on the nodes of the infrastructure. The selection problem we address is framed as an optimization problem of the quality of service, which evaluates at run-time the optimal binding to concrete services as well as the trade-off between the remote execution of software fragments and their dynamic deployment on local nodes of the computational environment. The final deployment takes into account quality of service constraints, the capabilities of the physical devices involved, including their performance and energy consumption, and the characteristics of the networking links connecting them.