I'm sorry to bust the fun on this, but I would never rebuild this device (or anything other with radio capability)... simply because of the costs of certification which is mandatory for any RF transmitter.
The problem with a lot of home automation and internet-of-things stuff is proprietary lock in, either from new young companies that could be dead in 18 months or established monsters looking to slurp your data. Having an open source foundation is the only viable long term approach to something that needs to live as long as your home infrastructure. If this lot cease being able to make the boards then at least, in principle, you can. (Or more likely someone else can pick up the ball without any license cost). If you've built around a Nest and that stops being made then you're screwed.
I have to admit I don't think either in extreme is going to work. You need the combination of standardisation with non-cloud dependency but also commercial viability. i.e. an Android of Internet-of-Things without Googly influence is needed, but without a rich backer it's hard to see where it will come from.
This is the real reason the GPL has a sort of built in success limit for end user deployments, and why so much of the code in Android is Apache or BSD licensed.
(As I understand in some countries you need to get a license to sell wifi devices - some Arduino's have issues with this - but not to use or build them).
If so, this is incorrect. There is no FCC certification required for low-powered, non-licensed devices like this one if you don't sell or market them. You can run up to 5 such devices without any certication. See page 3 on:
http://transition.fcc.gov/Bureaus/Engineering_Technology/Doc...