The more I think about it, the more I think it makes sense that we're going to be seeing the introduction of some Siri APIs at today's Apple event. Let's consider the evidence, and you let me know if you think I'm crazy.
First off, there's the invitations to the event, which references Siri both in the "Hey Siri, give us a hint" text, and the new iOS 9 Siri interface design. That's pretty weak evidence, admittedly, but Apple does have a history of hiding little clues in these invitations.
Second, we can all just assume at this point that we're going to be seeing new Apple TVs at this event, yeah? And that we're going to be able to play games on it? There seems to be little doubt at this point. And if that's true, that means new developer tools. At the very least, we'll need a way of simulating on the new device, deploying builds to it, and distributing to the public. Some new TV-centered APIs definitely wouldn't seem out of the question if this is the case.
Next, another rumor very popular rumor is that the new Apple TV is going to come with a new remote with a built-in microphone, and support for "Universal Search" of content. So say you're a developer at Netflix, and you're building the Netflix Apple TV app, how would you have your content shows up in Apple TV's Universal Search without an API? Same goes for Hulu, and Amazon, and YouTube, and Plex, and ESPN, and HBO, and so on and so forth.
Is Apple going to open this feature up to only a select few partners? I don't think so. That would be unmaintainable for what I would imagine is going to be a very popular new platform. Better to build an API that works for anybody who wants to use it.
And finally, Apple likes to roll out entirely new APIs in a limited fashion. It's how we got things like background processes and app extensions, and I believe that it's how we'll get Siri APIs. A Universal Search API would give Apple the ability to dip their toe into the world of letting developers get their hooks into Siri without jumping in all at once.
So, I don't know. This is all obviously speculation and maybe I'm reaching a bit. But it's fun to let yourself get carried away with these things and see where your imagination takes you. Especially for an event that we know so much about already.