It turns out that using some common electronics and an open-source Wi-Fi development board called Spark Core, you can build a Nest-like device of your own for about 70 bucks.
The team at Spark did exactly that, creating a mostly fully featured Nest-like thermostat powered by their board. Then they released full instructions on how you can do the same.
So who knows what you'll do with an open-source Nest? As Spark writes, "We're not saying that you can build a $3.2 billion company in a day. But we are saying that you can build a $3.2 billion company..."