Definitely still a work in progress. I'm still working on the power supply and physical connections for now. I spent some time experimenting with the software aspects, triggering the relays, reading values from the temp/humidity sensor, and in general learning how the RPi works. I also set up several SNMP capabilities: read adult presence served up from my firewall, read outside temp/humidity, send indoor temp/humidity to my Cacti server.
I plan to use the heat pump's own 24VAC thermostat transformer to power the Pi through the rectifier, filter cap, and voltage regulator. The 24VAC transformer I bought from Radio Shack to test and tune the regulator circuit (before hooking up to the heat pump) seems to be bad, though.
It will determine whether my wife and I are present using our smartphones. If we are both gone, the tstat will switch to a lower-energy mode, keeping the house above the pipe-freezing level in winter, and below 90degF in summer. It will also provide information to Cacti allowing me to correlate indoor and outdoor temps and humidity, as well as see operation modes and runtimes. It'll also have a schedule interface.
The heat pump is fairly standard: single-stage cool, single-stage heat with aux and emergency heat. Thermostat hooks up via 7-wire interface (2 power @ 24VAC + 5 triggers: fan, compressor, refrigerant reverse, aux heat, emergency heat). I'm kinda hoping I can add an 8th wire to get status back from the indoor unit (which I think simply indicates when the blower is running, but I don't know); it will help with triggering things at the right time and with tracking energy usage. The fan tends to run for a few minutes even after all relays are opened.
That's the basic plan for this project. Now I just need to find time and fix my AC transformer.