Even if you hold your laptop up to your face like a flip phone, talking into your computer is lame. You want to use Google Voice like a real phone—with buttons and stuff. We'll tell you how.
This Line2 app was just approved after a 3 month delay, partially caused by the Google Voice shenanigans Apple pulled back in July when they pulled out all the Google Voice apps from the App Store. This is really similar.
Apple may have booted the official Google Voice App and GV Mobile
Sean Kovacs' excellent GV Mobile iPhone app
Google Voice