Wireless sensor networks are a hot topic in research - this article from Roland Piquepaille - Less power needed for wireless sensors - has some good info on sensornets:
In recent years, wireless sensor networks have been deployed in inaccessible and unwired places, for example to monitor wildlife activity in wilderness parks. But all the sensor units are battery powered. So it's crucial to use as less power as possible to operate these sensor networks, or "sensornets." Now, Californian researchers have implemented a new communication protocol for wireless sensor networks which is ten times more energy-efficient than existing protocols. In current sensornets, each individual unit needs to be active at least 30 minutes everyday while this new protocol requires less than two minutes of activity per day.