Aside from pressing the home button to activate Siri, this Siri hack reported by IntoMobile is absolutely thought-controlled. The developers who initiated Project Black Mirror recorded activity of brain waves with ECG sensors, matched them to pre-saved patterns on a MacBook and then direct the matched commands to a chip that translates them to Siri. The developers linked about 25 brain wave patterns to different functions controlled by Siri, and they hope to bypass the need to press the home button with a completely automated solution. You can see on the video above how a developer initiates a call using Siri without pressing the Home button.
If the developers behind the Project Black Mirror achieve their end goals, in the future you can just think of a task you want your device to perform.
There already exist similar projects, but they are not based on Siri. InteraXon demonstrated its thought-control technology for converting brain waves to digital signals at CES 2011. The video below demonstrates the usage of this technology.