I think that once Apple gets Siri a bit more polished they will open it up to developers for limited functionality. I don't know if they would open it up completely. I didn't realize until after launch that Siri handled the processing on the server side. I can't imagine full integration into every app. Seems like overload and overkill.
I mean really, why would you need to voice activate the opening of an app? If you are going to interact with that app, it is just as easy to open it up normally.
I see Apple's "opening" of Siri to specific types of apps that mesh well with voice controls. For example map apps, translation apps, etc.