Well my current BT headset will pass though a voice call request if I hold the button. I'm guessing one could hook that into the 'home' button signal for iOS to pull up Siri, and coding Siri to accept 'my_name "to" <contact>' as a request to speed dial contact, well that is just a bit of coding as they say.

My guess is that Google could do it trivially by oneboxing the Google Now API (you know how they intercept a search for recursion to return a specific result).

Now building a small speaker phone gizmo the size of the Star Trek com-badge is also pretty doable if you don't mind hacking on a BT headset.

So the pieces are there. I hope someone builds one and submits it to Hackaday. I'll have to go look and see how hackable the souvenier communicator I got at the Star Trek Experience is.

I think you can already do this. I had a Blackberry a few years ago that would go into voice command mode when you pressed the button on any Bluetooth headset. Presumably this is standard behavior (though I can't actually find my headset to test with the Nexus 7).

With the standard in place, it's a simple matter of 3D printing a suitable enclosure and having it press that button.