Urban environments are increasingly filled with digital display systems that are inflexible, flat, bounded, high-resolution, and unresponsive. In this paper, we explore the potential of physically instantiated pixels that enable flexible, reconfigurable, unbounded, low-resolution, and responsive urban displays. Urban Pixels are nodes in a wireless network of physical pixels for urban spaces. Each pixel unit includes a microcontroller, RF transceiver (433 MHz), LED module (ten bright, white LEDs), rechargeable Li-Ion battery pack, IR sensor and renewable energy source such as photo-voltaic cells. Two acrylic half-spheres (4-inch diameter) protect the components from the elements. No additional wiring is needed for communication and the units can be mounted individually to any surface. A small-scale prototype network of fifty Urban Pixels was displayed on a façade of Eden Court Theater in Inverness, Scotland from June 1 - June 7, 2008. The public was encouraged to change display patterns via SMS or to interact with individual units via flashlights. We observed and informally interviewed theater guests and passers-by interacting with the façade for several nights. Based on these results, we outline an exciting problem space for designing displays and lighting systems in cities.