Advocatus Diaboli sends this excerpt from an article about the data collection capabilities of the Nest Protect 'smart' smoke alarms, and how they could become a privacy concern:
Consider that each Protect is packed full of sensors, some of which are capable of much more than they're doing right now: From heat and light sensors to motion sensors and ultrasonic wave sensors. This simple little device could scrape an incredible amount of data about your life if Nest asked it to: From when you get home, to when you go to bed, to your daily routine, to when you cook dinner. Now imagine how a device like that would interlock with another that you keep on your wrist, like the forthcoming Android Wear. Together, they would create a seamless mesh of connectivity where every detail of what you do and where you go is recorded into a living, breathing algorithm based on your life.
Neither Nest nor Google has stated any intention to turn Nest's hardware into more than it is right now. Protect is an alarm, the Thermostat is a thermostat. But as Google ramps up its vision to connect every aspect of our world, from Android Wear to its acquisition of a company that specializes in high-res, near-instantaneous satellite imagery of Earth, it's easier than ever to see why it would cough up billions for a company that has installed hundreds of thousands of Wi-Fi connected devices in the homes of Google users."