I have a OmniStat for my house, about $300, nice standalone thermostat with published serial protocol.
If you have an Omni security automation system to go with it you get IP access (again published protocol with a few iPhone apps) and the Thermostat can change temp based on security system state(away, night, etc.) or you can write automation code on the Omni to do whatever or use the IP protocol to do eve more from a real computer.
You don't want PC controlling the A/C directly, the OmniStat will continue to function without it's smarter controllers if need be, same with security, thats why I like my Omni system, it's a rock solid "lower brain stem" for my house while the PC is the "higher brain functions" that can go offline if need be without too much disruption.
If you have an Omni security automation system to go with it you get IP access (again published protocol with a few iPhone apps) and the Thermostat can change temp based on security system state(away, night, etc.) or you can write automation code on the Omni to do whatever or use the IP protocol to do eve more from a real computer.
You don't want PC controlling the A/C directly, the OmniStat will continue to function without it's smarter controllers if need be, same with security, thats why I like my Omni system, it's a rock solid "lower brain stem" for my house while the PC is the "higher brain functions" that can go offline if need be without too much disruption.
http://homeauto.com/Products/Omnistat/Omnistat2Products.asp
Supports 2-stage cool/ 3-stage heat too, but really needs a common wire for power, no batteries.