We present a hardware and software platform for rapid prototyping of augmented sensor network systems, which may be temporarily connected to a backend infrastructure for data storage and user interaction, and which may also make use of actuators or devices with rich computing resources that perform complex signal processing tasks. The use of Bluetooth as the wireless networking technology provides us with a rich palette of Bluetooth-enabled commodity devices, which can be used as actuators, infrastructure gateways, or user interfaces. Our platform consists of a Bluetooth-based sensor node hardware (called BTnode), a portable operating system component, and a set of system services. The paper gives a detailed motivation of our platform and a description of the platform components. Though using Bluetooth in wireless sensor networks may seem counter-intuitive at first, we argue that the BTnode platform is indeed well suited for prototyping applications in this domain. As a proof of concept, we describe two prototype applications that have been realized using the BTnodes.