I don't know how this is going to work. If you and I both make a Twitter app. And you ask Siri to tweet something, how is she going to know which app to send the request to?
And how is she going to decide which messages are to be considered for tweeting to begin with. If I make an app that checks hockey scores and you ask for the baseball game, how is she going to know the difference between the two and that my app can't do baseball scores.
It all gets very complicated very fast. I won't be surprised if API (if any) is very limited. So don't get your hopes too hight.
Android's had this solved via intents for years now.
Applications register to say "Hey, I want to handle this action". When the user initiates that action, if there are multiple apps that can handle it, the user is prompted to choose which app to use to handle it, with the option to remember the choice.
And how is she going to decide which messages are to be considered for tweeting to begin with. If I make an app that checks hockey scores and you ask for the baseball game, how is she going to know the difference between the two and that my app can't do baseball scores.
It all gets very complicated very fast. I won't be surprised if API (if any) is very limited. So don't get your hopes too hight.