This is a hot idea...or maybe it's a cool product, either way, I love it.
About 14 years ago I installed my first programmable thermostat to help save on my heating and cooling bills. I have since replaced it with newer versions as they have become smarter and smarter. The problem has been that the thermostats are either hard to program or have few day-evening-night-weekend setting options, so they work on a fairly rigid schedule. This means I spend a lot of time overriding the settings that took me forever to program in the first place. To me, this looks to be the answer to all of this: a thermostat that learns how you live and works with you to maintain a comfortable temperature in your home, while saving energy.
Also, from a design point of view, it is a very attractive device -- clean and sleek looking.
Rather than have me go on about it, take a look at this video.
You can also visit the Nest site here Nest.com