Doesn't seem to be available for iPod Touch even though many of the features would work fine on it.
It appears number portability and iPod Touch support is not on their priority list. I have no clue why. With those two features, personally, I would be free from cell phone carriers.
1. Set up Google Voice to forward your GV # to your cell phone.
2. Install the Google Voice app to get push notifications for SMS messages sent to your GV #.
3. Give out your GV # instead of your cell phone number.
4. Change the forwarding rules at will -- you can even use the same number for multiple phones.