Ubiquitous smart environments and anywhere anytime Internet availability are pushing to the extreme demand for effective and standardized integration solutions in the Future Internet. Standardization efforts of extreme industrial relevance, such as the IP Multimedia Subsystem (IMS), have recently emerged to support session control and interoperability in all-IP next generation networks. At the same time, recent advances in wireless communications have motivated the development of Wireless Sensor Networks (WSNs) for low-cost and easy-deployable environmental monitoring. However, the IMS potential for new classes of ready-to-use, energy-efficient, and highly integrated services, including also WSNs, is still widely unexplored. The paper proposes an original solution to opportunistically use client devices, that roam in geographically sparse environments (such as a smart city), as WSN data harvesters. The primary design guideline is to exploit IMS and the recently released IMS presence service to effectively coordinate harvesters, by saving energy via minimization of unnecessary communications. The reported experimental results point out that our solution, compliant with standards and thus easily deployable, can significantly increase sensor battery lifetime with minimum impact on the freshness of harvested data.