The increase in physical resources for the next-generation of embedded computing devices, as well as in the efforts carried out by the scientific and research communities, is paving the way for Smart Infrastructures based on Wireless Sensor Networks. In such manner, not only open and formally defined Service-Oriented Frameworks are required, but also efficient middleware technologies that ease the development of new sensor-based services. From the cornerstone analysis of pervasive computing principles applied to Smart Spaces creation, this article presents the μSMS (micro Subscription Management System) middleware. This approach specifies and develops the notion of virtual sensor services created for Smart Environments over sensor networks from tiny in-network services based on agent technology. In the framework of the μSWN European Research Project, this architecture has been validated in a Smart Hospital real-world scenario using a healthcare virtual sensor service. Considering such purpose, medical status monitoring, location tracking and perimeter surveillance agent-based services have been developed. This study is concluded by a comparative analysis of the system considering memory overhead, packet delivery ratio, average end-to-end delay and battery lifetime as evaluation metrics. The results show a lightweight middleware implementation with good overall system and network performance.