The smartphone revolution has brought ubiquitous, powerful, and connected sensing hardware to the masses. This holds great promise for a wide range of research fields. However, deployment of experiments onto a large set of mobile devices places technological, organizational, and sometimes financial burdens on researchers, making real-world experimental research cumbersome and difficult. We argue that a research infrastructure in the form of a large-scale mobile phone testbed is required to unlock the potential of this new technology.We aim to facilitate experimentation with mobile phone sensing by providing a pragmatic middleware framework that is easy to use and features fine-grained user-level control to guard the privacy of the volunteer smart-phone users. In this paper we describe the challenges and requirements for such a middleware, outline an architecture featuring a flexible, scriptable publish/subscribe framework, and report on our experience with an implementation running on top of the Android platform.