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.
Google's been on the record for several years as hoping to support this in the future.
iPod touch and iPad users account for a little under half of all iOS devices. Still a huge portion of the market to miss.