I've just built something very similar to this last weekend -- For around $43/sensor (Raspberry Pi Model B, DigiSpark, and 1-Wire Temperature Sensor) I made 20 of these for my home, farm, and hackerspace for temperature logging. I did this because we're getting another 'polar vortex' next week and the cows don't like it if it's colder than 20 degrees out.
This allows me to measure the temperature inside, outside, and get the relative humidity (not nearly as accurate as the $20 honeywell sensor that they're using, but, it's close enough for my needs). I then built a simple website using mrtg (for temperature trending) and a ruby script that checks the temperatures versus what the set points are and mounted the raspberry pi's in various locations around my places.
My "Controller" nodes are a beagleboard with a 4 or 8 channel relay board attached that allow me to turn on or off the individual controls on the furnace. It works well with my two stage heat pump and fan at my home, but, I need some work to get it 100% at the hackerspace and at the farm.
I mainly did this because I needed something that allowed me to cover more rooms than the Nest (and I'm adding duct dampers and fans to my heating system, so I can selectively heat and cool more rooms to better temperatures).
When you get a chance, would you be kind to write a "how to" blog post about how to build and wire one yourself, so others without any clue about how to do this can have a shot at learning and doing it themselves?
The biggest thing that I had against using the Pi's GPIO pins were that they're really unforgiving, and only ran on 3.3v. My One-Wire sensor (http://datasheets.maximintegrated.com/en/ds/DS18B20.pdf) Needed 5 Volts, and that's something that the DigiSpark gave me with no issues. I literally soldered pins 1 and 3 to the +5 and Ground pins on the back of the digispark, pin 2 to the I/O5 on the digispark, and ran a 4.7K resistor across the front from +5 to Pin 5 for my pull up resistor -- stupid easy to do and a squirt of epoxy made everything electrically tight.