The proxy server was built by a developer named plamoni who has posted a video that shows him controlling his thermostat with Siri. It responds to commands such as, "What's the status of the thermostat?", or "Set the thermostat to 68 degrees", or even "What's the inside temperature?"
On his GitHub, plamoni thanks Applidium for their work on cracking the protocol and posts instructions for those who are interested in trying the proxy server. Please note that this will only work while connected to your home network until a jailbreak is released for the iPhone 4S.
Take a look at Siri Proxy in action below...