More complicated clients, one which monitor updates from the steward looking for changing conditions, and reacting to them on behalf of the user by asking the steward to perform actions, are entirely possible. Technically speaking these are intelligent agents working in a multi-agent system with shared goals of making life more convenient for the user. We like to think that we're implementing magic.
Reads a light and temperature sensor and exposes their values (JSON) over a simple HTTP server running on an arduino (with ethernet sheild). Also exposes the value of a relay which can be updated over HTTP.
This article will help you implement a simple project that will require limited hardware and you can develop an app that can monitor your room or home temperature using Windows Phone.