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.
Number portability in this context means the ability to use an existing number with new service, not the ability to use a new number which points back at your existing number.
Google's been on the record for several years as hoping to support this in the future.
To my understanding, it doesn't implement calling functionality -- you have to have a number for the Voice service to call to. It isn't a VOIP client like Skype, so I wouldn't expect this to support the iPod touch any time soon -- unless you were planning on using it to forward to a Skype number or something like that.
It's really more of a telephony abstraction than a client.
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.