Siri will be open to 3rd party devs IMHO. User will say to Siri, "[App Name], [command]."<p>"New York Times app, read me the 10 top stories."<p>"Pandora app, play Sugarland station."<p>Each developer can list a series of actions = verb + object. When the user installs the app, Siri will pick up all the actions and they will be available to the user. Again, it must be initiated by the user saying the app name first. I can see two main usage cases:<p>1. Launch app into particular action - for example, "Pandora app, play Sugarland station." This opens Pandora and plays Sugarland station. Pretty straightforward. Devs just determine the verb + object and what Siri does with the action. In this case, Siri launches Pandora and sends over the verb "play" and the station "Sugarland," and Pandora knows what to do with it.<p>2. Read/show data within Siri. So, in this usage case the app doesn't launch but rather gives Siri data. For example, "New York Times app, read me the last 10 stories." Siri sends "read" (verb) and "last 10 stories" (object) to New York Times app. New York Times app gets that action and does an API call to the server to return the 10 top stories and gives it to Siri to read. Siri reads them to the user.<p>#2 usage case is pretty mind-blowing if Apple can pull it off. Because it's taking apps and pulling off its cover and stripping them to it's core services. Users don't have to launch the app to get certain data. App become more services that Siri can access. Of course, the user can still launch the app and do other things in the app via touch. But the app has a voice interaction interface and Siri is the master of that.
First of all, wether or not Apple decides to open an API to siri and wether or not it's possible are two separate things. None of his arguments are very convincing to me. Managing a 3rd party ecosystem is a hard problem but not <i>that</i> hard; especially for Apple who really can call the shots however they see fit. Here's a scenario: all 3rd parties can plug into the siri API but they need approval and can be kicked out for whatever arbitrary reason Apple picks. That would suck for developers (see marco's post earlier) and yet most developers would jump through hoops to get into there.
The only major issue I can see is that Apple favored apps will always take first pick.<p>If you want to say "Tweet blah blah", and use something that isn't the official Twitter client, you're probably shit out of luck. You'd have to say the app's name or something of the sort.<p>Another example is that Reminder will take precedence over any other third party todo app.<p>Otherwise I think there are ways around the whole 3rd party issue with a strict approval process and having developers somehow interpret their own text commands and provide some standard feedback about what Siri's response should be.