I was looking through the VoIP app section inside the Apple iPhone store and I found something I had missed. It seems somewhere along the way Acrobits released an authorized iPhone/iPod touch and iPad compatible application for Gizmo5.
Basically, what it does is give you a softclient that works on the iPhone that connects to your Gizmo5 account (that is if you had one before Google bought Gizmo/SipPhone.)
Now here's the interesting play--if you've activated your Gizmo account to connect to your Google Voice account your calls ring right through as a pure SIP call. And with background notifications enabled, you can be doing other things on your iPhone and receiving calls in real time too.
And who said GoogleVoice didn't work on your iPhone?