In 2011 Apple introduced Siri, their virtual personal assistant, with the iPhone 4s, and two years and two versions of iOS later, including this year's iOS 7, and no public application programming interfaces (API) have been surfaced for developers. That means customers like you and me still can't use Siri with App Store apps (aside from Apple's own Apple Store app). There are likely both business and technical reasons for that...
Apple currently makes partnership deals for Siri, including Microsoft for Bing search, Fandango for movie tickets, Open Table for restaurant reservations, and more. If any app can tie in, those deals lose all value. Also, if multiple apps of the same kind are installed, competition and collision need to be carefully handled. Likewise, private APIs can be messy as hell. Public APIs need to be neat and clean. If Siri needs to be modularized and made it's own thing for it to work, that takes even more time and effort. And let's be frank, it would have to be rock solid, because Siri already has a hit-and-miss reputation with users, and Apple services APIs, like iCloud, with developers.
I’ll bet on an API later rather than sooner. If only because the success of Siri is determined upon it generating the best possible results and the ranking of those results must be determined by the plug-ins it uses. Once you allow plug-ins that potentially lie then you’ve ruined the entire Siri experience.
Read the whole thing, especially the dramatization, and for a deep dive, I recommend the discussion between Guy and Grant Paul (chpwn) on Debug.
Getting Siri on-device, perhaps with natural language and contextual coprocessors like Motorola is doing with Google Now all seem like things we should see with Siri before an API.