For some reason, computer controllable thermostats are pretty freakin' expensive. I found a reference to a 1-wire thermostat in this(mirror) sample senior project on home automation. It turns out that Dallas Semiconductor put one together a while back as an application for their TINI platform. (web-application server on a chip). The write-up has since vanished from their site, but I found it thatnks to archive.org. The thermostat used to run about $50, and a similar model still seems to be produced. The 1-wire interface is pretty simple - Maxim's TINI board to control it: not so much. Just using the 1-wire interface with an inexpensive thermostat and controlling from a PC seems pretty viable to me. Just in case, I mirrored the 1-wire interface schematics here.
http://www.hackaday.com/2007/10/24/1-wire-thermostat-control/