Siri on the Mac (via OS X 10.9) would be yet another indication of the coming collision course between mobile and desktop devices. Voice assistant software arguably has less value on a laptop/desktop with a full-size keyboard and mouse/trackpad than on a smartphone or tablet. But since Mac users will soon be able to do hands-free, natural-language searches via Google Chrome, it behooves Apple to bring Siri -- warts and all -- to the Mac as quickly as possible and (once again) limit Google's encroachment.
A related thought: If Siri comes to the Mac, can touchscreens be far behind? Apple is almost certainly studying Microsoft's blunders with the touch-oriented Windows 8 Modern UI -- and probably looking for better ways to meld touch input with a traditional desktop OS.