The “Samantha” voice in OS X Mountain Lion is the same as the voice of Siri in the United States. To get Siri’s voice on Mac OS X, you will have to download it first. It’s about a 500 Mb download. After the download is complete, open System Preferences > Dictation & Speech > Text To Speech > System Voice on your Mac.
Select ‘Customize’, then select ‘Samantha’.
For now, Siri, or Samantha if you like, can only be used for turning selected text to speech. The default keyboard shortcut for this is Option + Escape. You can use the same keyboard shortcut to make Siri stop talking.
Here’s an example of the voice of Siri on OS X: