Quick update.. A possible alternative to the grooveip forwarder that I just found is this Auto AP app https://play.google.com/store/apps/details?id=net.geekherd.autoap
The idea is to setup google voice to forward to both your cell# and gchat/grooveip. To avoid the double ring, this app will enable/disable cell antenna by putting your phone into airplane mode automatically based on wifi signal. It'll even start/stop grooveip as well.
As a (huge!) side benefit, you'll save lots of battery by not having your cell radio on all the time when you're at home/work with a wifi signal.
I've done some basic tests with it and it does seem to do what it says. I haven't done much real world testing of this but I'll update.