I wondered the same thing when I first started using Google Voice with my Android while at home and with my phone showing a WiFi connection. Turns out that when you make a Google Voice call on an Android device, Google Voices calls into a direct access number where the server that then forwards your call to the number you dialed. So you are actually still making a conventional cell phone call, it is just to a different number, even when your device is showing a WiFi connection. This struck me as odd since I could sit at my desk and make free Google Voice calls on my laptop over WiFi, but the Android phone in my pocket makes me use talk minutes.

You can, however, make calls over WiFi with an Android using Google Voice, but you will have to use an additional app. There are a few choices out there. GrooVe IP is a newer one that has good reviews, and there are more mature apps such as Sipdroid and CSipSimple. It may take a little effort to get everything configured with some of these apps, but after you do, you can use Google Voice directly through your WiFi connection without using either your data or talk minutes. Depending on where you make most of your calls, you may be able to lower your minute plan and save a little money each month. It's definitely worth giving it a try, and at worst you will only be out $5.