How do you use Google Voice instead of built-in text message with Siri on iPhone4S? I would like to use the Google Voice app instead of the built-in Text Messages with Siri on the iPhone 4S.

Is this possible and how?
No, it is currently not possible for Siri to use other apps than the ones it already integrates with.

Source: Daring Fireball (paragraph 9, or ⌘F “there are parallels”).