WorldChanging has a story about a “Czech company called Bladox, which manufactures small circuit boards and software to plug into old mobile phones.”
Bladox makes boards and software allowing for a wide variety of inputs; better still, their applications are all licensed under the GPL. With this hardware, pretty much any kind of low-power sensor system (temperature, location, motion, air quality, etc. etc.) could be hooked up. Rigging a mobile phone as part of a sensor system takes care of one of the critical elements of any such device: communicating results. As long as there’s network coverage in the sensor location, the results can be sent to anyone in the world with a phone. And the sensor-phone would still have a phone number, allowing for calling in to get results as needed. There are numerous solar panel rechargers for phones, so keeping the battery topped up wouldn’t even be a big issue.
The vast majority of mobile phone users discard their old phones long before they stop working. The fact that many mobile services “lock” the phones, making them only work with that particular service (unless unlocked, a sometimes tricky process), makes such a wasteful practice almost inevitable. There are millions of completely functional but ostensibly useless phones out there; you probably have a few sitting around your home (I know I do). While recycling is possible (and far preferable to just throwing them out, given the toxic metal content), there’s something particularly appealing about reusing the phone in novel ways.