Unfortunately when the official Google Voice app for iPhone was released, it was unable to be installed on iPod touch. However, you can now get the app up and running on your iPod touch, as long as it is jailbroken, that is. (Thanks to Mac OS X hints reader pet3rp4n)
Of course the calling will not work on the app, as your iPod touch does not have the cellphone signal necessary to make it work. However, all other functionality of the app should work as expected, including free text messaging, voicemail transcriptions, syncing contacts, and push capabilities. If you are brave enough, a step by step instructional guide follows:
1. Download the Google Voice app for iPhone from iTunes.
2. Right click the Google Voice app within iTunes and select ‘Show in Finder’ (‘Show in Explorer’ for Windows users).
3. Copy the app file to an easily accessible location of your choice and change the file extension to .zip
4. Now open or extract the .zip file.
5. Naviagte to /Payload a re-zip the GVDialer.app (in Windows it will appear as a folder, while in Mac OS X it will be a single file)
6. Now, use SSH or another method to connect your iPod touch to your computer over WiFi.
7.Download ‘iFile’ from Cydia and open it.
8.Select the WiFi Signal icon from the bottom of the iFile app. This will give you an IP address
9. Copy the IP address from the previous step and paste it your web browser (it will look something like this- http://22.214.171.124:10000)
10. Within your browser, navigate to /Applications folder and upload the GVDialer.app you zipped up in step 5.
11. Now, unarchive the file using iFile’s unarchive feature
12. Lastly you will have to apply new settings to the GVDialer.app folder by pressing the blue arrow next to it. Settings that should applied follow: Apply hierarchically » ON; Owner » root; Group » wheel; User » Read,Write,Execute; Group » Read,Execute; World » Read,Execute.
13. Now, reboot or respring your device and you should be finished! Let us know how it goes in the comments.