Once the folks at Applidium had cracked the Siri protocol, it was only a matter of time before the first mods, exploits and hacks started to leak out. Because Siri relies on Apple's servers to deliver results, the limitations on what it can and can't do are entirely controlled at the server end. However, with the way Siri communicates with the data center now fully understood, a developer called Plamoni has managed to create his own custom handlers, using a proxy to catch terms and act upon them accordingly.
Using this method, Plamoni can now control his Wi-Fi thermostat via Siri, with natural speech parsed as you'd expect, and Siri responding as if it was connected directly to Apple. More commands are being added daily, with a Twitter module already included. Best of all, this doesn't require a Jailbroken device, just the installation of a fake root certificate onto the phone. The proxy software is available on GitHub along with instructions, though for now the developer warns that it's not for the faint-hearted, and whether it'll play nice with your Nest is another matter entirely.