Finally, Google Voice is available on iPhone. So I trust it is also OK for iPod Touch.
They said:
With this native app, you’ll continue to have access to all the major Google Voice features on your iPhone, like:
* Cheap rates for international calls
* Free text messaging to U.S. numbers
* Voicemail transcription
* Display your Google Voice number as caller ID when making calls
I can not wait anymore. I will go home to try it on iPod Touch.