Operating systems for reconfigurable computing are becoming an attractive field of research. They provide a well-defined programming model and a run-time environment, which greatly simplifies the development process and management of reconfigurable applications. One of the main challenges for the design of such systems is to provide both powerful and efficient abstractions to deal with the complexity of the integration between hardware and software domains in general, and the special features of the reconfiguration process in particular. The contributions in this paper try to give solutions to both problems, first taking a distributed system approach based on system-level middleware, and second providing transparent reconfiguration as one of the advanced services provided by the middleware.