A large range of application domains, from real-time embedded systems to grid-computing applications, now requires distribution. This trend implies definitions of new or tailored distribution mechanisms dedicated to specific applications and puts a strain on current middleware architectures and development. Middleware intends to separate an application from variations in hardware and operating systems. This new interoperability problem itself is serious industrial issue. We call this the middleware paradox. Next-generation middleware should be versatile enough to instantiate the exact required mechanisms of different distribution models. Middleware components that depend on a specific distribution model should be limited to application-level components or to protocol-level components.