I hope that as developers we're able to take advantage of this conversational style. At the moment, WP apps can hook into the rather limited "Tell Me" system that lets the user open various endpoints of an app via voice commands, and only using pre-defined parameters.
It would be awesome if the user could pass in any arbitrary data via voice commands (devs could make custom scheduler/todo list/note-taking apps that can receive data from Cortana), and even more so if voice commands could launch background tasks instead of just the entire third-party application (you could check into Foursquare without actually opening the Foursquare app and disrupting whatever you're doing).
I think that Twitter might be entwined enough into the OS such that it'd get permissions that other third-party devs wouldn't. I would totally love to be wrong though.
I actually started watching the live stream after they demoed Cortana but going back now and seeing Belfiore demo Cortana:
- He added a show to his queue in Hulu but it stayed in the app
- He said "what's up with Terry Meyerson" and it went straight to Terry's facebook profile.
And I realize that FB has some deep integration within the OS (just like Twitter)
Did you try Wit.AI? [1] It's cross platform and totally open: you can create any voice command you want by just providing a few examples of what you expect. Then the API turns speech (or free text) into JSON.