In this paper we introduce a novel scenario for environmental sensing based on the combination of simple and cheap RFID-based sensors and mobile devices like mobile phones with integrated RFID readers. We envision a system, where the mobile nodes cooperatively read sensors installed in the environment as they pass by and transmit the data to a server infrastructure. To achieve quality requirements on the one hand, and on the other hand efficiency in terms of communication cost and energy consumption, this paper presents several algorithms for coordinating update operations. Mobile nodes form an ad-hoc network for the cooperative management of requested update times to meet the desired update interval and to avoid redundant sensor reading and collisions during read operations. Besides this decentralized coordination algorithm, we also show a complementary algorithm that exploits infrastructure based coordination. By extensive simulations we show that our algorithms achieve a high quality of sensor updates where nearly 100% of the possible updates are performed. Moreover, the algorithms achieve a very high energy efficiency allowing for several hundred hours of operation assuming a typical battery of a mobile phone.