MacRumors has been seeing builds of OS X 10.9 in site logs since November, which is also when hints of Siri integration for Macs began appearing.

A recent Apple job posting (via TechCrunch / AppleInsider) reinforces the rumor that Siri might be bundled with the next version of OS X. In its listing for a Siri UI Engineer, Apple specifies that candidates should possess "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.

 Apple Job Posting Hints at Siri for Macs in OS X 10.9