Apple has been seeding build of OS X 10.9 from November, in which hints of Siri was found
Now App there in a Job posting in found in Apple’s site which calls for a “Siri UI Engineer”. This job posting seems to confirm the Siri in OS X rumour, as Apple wants the person to have “Familiarity with Unix, especially Mac OS X” and a “Passion for the Macintosh platform and writing simple, elegant software that is easy and fun to use.”
The listing, which does not include a specification for experience with iOS aside from knowledge of Apple’s development APIs, states that the engineer will be in charge of implementing the content that appears within Siri’s conversational view. The position will also require collaboration with other Siri teams.
This is a broad-ranging task – we take every application that Siri interacts with, distill it down to fundamentals, and implement that application’s UI in a theme fitting with Siri. Consider it an entire miniature OS within the OS, and you get a good idea of the scope!