Why would you want an open source Nest? Is there a market demand for something which is uglier, harder to use, takes more time to install, and works worse? Do you also build your own toasters or automobiles?
http://i.imgur.com/8fFkdQ9.jpg
In that dashboard of mine, you see devices from 6 different companies that I've integrated. 3 of them required reverse engineering some kind of website for remote control, even though all that website does is pass on a command from my home network back to my home network.
This would break if the website goes down, the website rate limits me, the website blocks my IP, or the website owner decides to sue me under CFAA because their TOS probably didn't authorize remote control outside of their official apps. Nest is one of those websites. There's no official public API.
I consider alternatives where I could just talk to the device directly a selling point, and it has nothing to do with privacy. I don't care about Google having my temperature history at all.
But for me, it's the tinkerer inside. I would rather build one of these than buy a nest.