I'm inclined to agree that a home thermostat is probably not the best place to be hacking around--especially, but not only, in cold climates--although this is really a comment on DIY generally rather than open source. Heck, I'm not even inclined to trust a Nest over a more traditional thermostat.

There are plenty of opportunities to fiddle around with sensors and other "smart home" gadgetry. But there are some areas where it makes sense to be very cautious.