Siri for Macs? If this rumor is true and if it's like Siri for iOS, I guess it will only work for people who purchase Macs at or after the release of 10.9.
In any case, I'm curious for what new features this may bring. I see Maps is rumored as well, but I can't imagine much use for this on a Mac when there are websites that can do the same thing, unless they add cellular or GPS to the MBA (probably not).
You can currently get the "Samantha" voice for your Mac, of course not with Siri functionality. I use it to speak my alerts.