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.
Google's been on the record for several years as hoping to support this in the future.
It's really more of a telephony abstraction than a client.