It looks as though Google Now may soon be gracing Mac systems. Code has been ported over to the Chromium project for Mac OS X and seems to indicate that Mac OS X will soon be getting access to Google Now. This same code was introduced into Chromium for Windows and Chrome Canary, but was limited to Windows OS.
The Google Now system is believed to rely heavily on the notification center. According to Google Evangelist Beaufort, the notification center for Mac is still "under heavy development." The Windows version of Google Now still isn't fully implementede; you need to know the secret URL to enable it.
It shouldn't be too much longer before both systems get access to Google Now, though the usefulness of it may be slightly less as you won't be using it on a constantly mobile device.