Google Voice is not VoIP. It WILL use your minutes. Unless you have fav5 grandfathered in your contract or AT&T (you'll loose 3G) A-List, you are SOL.
I just moved to T-Mo on the 1st and moved in with 80/month contract free unlimited everything plan so I am ok. Before that I used HTC Magic (Rogers) with google voice and A-List on AT&T.
But to recap, you will need minutes. Google voice is more of a front number at the moment. Not a VoIP service as of right now. With purchase of Gizmo, it may turn that way later but for now, you talk, you use your minutes.