I did something similar, but I have two heat pumps for a/c and heating, so I have two thermostats. Additionally, I have peak period billing from the power company so I had some different goals. First, I wanted to make sure that the two of them didn't come on at the same time and shove the peak usage way up. Next, I wanted to heavily limit their use during peak period by pre-cooling the house before the peak period started. Additionally, I wanted to move the cooler air from one part of the house to another by using the fans to recirculate the air; once again, one unit at a time to keep the peak amount down. I succeeded.
These guys are web enabled using an ethernet board and can respond to network commands. Of course I have to store the settings in eeprom to allow for power failure and I get my power for the devices from the 24VAC provided by a transformer in the air handler. To solve the problem with time-of-day (a really big deal when you have peak demand billing), I have a house clock that gets time from a GPS chip and provides it over wifi as well as XBee to sync all the various clocks around the house. This way the thermostats know exactly what time it is. A real time clock could work also, but I wanted to experiment with the GPS and web and stuff. Then, I built a central device to control them as well as other devices around the house. So the combination can do pretty much anything I want them too.
For example, I have the controller turn the temperature in the house down to 74F at 10AM and cool stuff off before turning them completely off for the peak period from noon to 7PM. Then, it turns stuff back on and cools the house back down. I have different tactics for the winter and just typically turn them off for spring and fall. I live in the desert, so cool is a major concern and I have adapted to the heat so a temperature above 80F is not a problem for me.
I put construction details, schematic and pictures on my blog at http://desert-home.com
or if you want to go direct to the thermostat, it's at http://www.desert-home.com/p/super-thermostat.html
. The sketch is probably a month or so out of date, but I'll get to that eventually.
To measure temperature I have a little XBee mounted to a wall wart that has a temperature sensor on it. This way I can move the thing around the house to see what's going on. I have one of these permanently mounted outside and another one that travels wherever I want to check the temperature changes. It's nice to have one when visitors complain about the temperature. I just plug it in and let them watch it themselves.
They've been in service for a little less than a year and I think I have most of the bugs worked out now.