It would be logical to assume that, at some point, the Siri voice assistant is going to make its way to Mac. And that could happen soon, in fact, as Apple has recently posted a new job listing for a Siri UI engineer that is familiar with Unix operating systems like OS X.
Siri is currently limited to the iPhone, iPad and iPod touch, yet this job listing makes no reference to iOS whatsoever. Apple has continued to bridge the gap between iOS and OS X over the past few years, so it would make sense for Siri to come to Mac at some point.
On a related note, the first developer preview of OS X 10.9 could be released this month. Apple has sped up the release cycle of OS X to be an annual occurrence, with Lion and Mountain Lion dropping in July for two consecutive years. The first preview of Mountain Lion came out last February.