ok, this will not be step by step down to the button push... but this is how you use google voice to make PURE VOIP FOR FREE (in the US and Canada) calls when using a 3g or wifi connection.
1) get a google voice number and verify your home phone or cell phone on the account.
2) go to sipgate.com (or some other FREE SIP provider) and register for a free account. Currently the only numbers are based in Californa but it doesnt matter since no one will actually be using this number.
3) install sipdroid (or some other SIP client) on your phone and configure it to handle your SIP provider.
(WARNING username/password needed is typically found under device settings for your provider, not what you use to log into the website)
4) register your SIP account to your GV account.
5) Setup GV to forward only to your SIP account and make sure your app is picking up the call.
6) Install on your phone the google voice application. (This will give you visual voicemail)
7) Install on your phone the gvoice callback app. (This will intercept your outgoing calls and let you place them via GV easily. This integrates with your phonebook as well as numbers you straight dial.)
8) Configure GVCallback for your GV account. (you should be able to make outgoing and receive incoming calls via VOIP at this point. The next step is optional.)
9) Setup a new phonebook entry that sets your cell phone number to forward to your GV number, and also removes the call forwarding. (for verizon it would be *72 your GV number and *73 to remove) With a quick access widget on your desktop you can now quickly set your phone into VOIP ONLY mode. If your outgoing call can not be made via GV the gvcaller app will let you know.
*edit* If I am way off base on the purpose of this thread, then I appologize. When I think of google voice I think VOIP