The real strength of Siri lies in its middleware infrastructure, that ties together more than 35 APIs. I’ve got a wealth of resources listed under my Meta-Guide.com project website. And, I’ve put together a comprehensive survey of “Artificial Intelligence Middleware” , as well as a webpage listing relevant “Application Programming Interfaces” .
In fact, Siri is based on a government sponsored (SRI) project called PAL (Personalized Assistant that Learns), and *some* of the PAL Framework  components are open source. (Actually, CALO, the Cognitive Agent that Learns and Organizes, was originally developed as part of the PAL project.)