In recent years, Wireless Sensor Network (WSN) has emerged as a highly important research area. Middleware for WSN facilitates development and deployment of a large number of applications such as smart environments, weather forecasting, bridge monitoring, health applications, etc. for sensor networks. But due to resource constraints, unreliability of wireless networks, and diversity in available sensor hardware, middleware for WSN presents a number of new challenges. In this paper, we try to find out and elaborate various challenges associated with the development of middleware for WSN. We present a comparative study of several existing middleware and how they address those challenges. In doing so, we point out the limitations of present generation middleware for sensor networks. We also illustrate how much more work needs to be done to make middleware for WSN suitable for general purpose usage in real world.