They've connected an iPhone 4S up to a setup that uses a MacBook Pro and an Arduino open-source microcontroller along with electrocardiogram pads that are attached to the head of the "test subject." The Arduino is set up with a program that recognizes Siri commands from the ECG pads and sends them to a voice synthesizer, which then feeds the commands into the iPhone's microphone jack.
The guys who came up with Project Black Mirror want to create a product out of this; perhaps a hat that you could wear and a sleeve that would provide the necessary input to Siri. As such, they're attempting a Kickstarter campaign to polish up the project and bring it to fruition. Check out their Kickstarter pitch below.
By the way, the guys are in need of help starting up a business in the US (and I'm busy and can't help them), so if you want to volunteer to give them a hand, be sure to visit their website (link above) and let them know.