Apple wouldn't allow any Google Voice apps on the iPhone. GVDialer which made one had to close its shop in October because of this. There is a second program (not made by Google) which is still around but your phone has to be jailbroken:
Sean Kovacs GV Mobile
Google Voice is not VOIP exactly. It is more of a personal line, visual voicemail, and a call through service. You could hook it into Gizmo 5's VOIP service, but after Google about Gizmo, they temporarily stopped taking new accounts.
There are all sorts of VOIP clients for the iPhone over WiFi.