I registered for googlevoice about a half an year back and have been using it to call Canada from US. It DOES consume my normal T-mobile minutes. Just yesterday I heard about the gmail phone call thing and found that calling with gmail phone call leaves a history in your googlevoice account. So does that mean using gmail phone call (calling from computer) would consume normal minutes from your phone registered with googlevoice number assuming these both services are same?
When are you consuming the minutes? In US (simplistically speaking), you pay when you are using your cell phone handset, both for incoming and outgoing calls.
If the call is not ringing your phone, but is using computer, I would say there would be no charge.
The backend architecture is the same. The Ggoogle Voice was an acquired company (Grand Central), which was slowly being integrated into Google architecture. Makes sense to not reinvent the wheel.
You don't have to use the cell phone either. Used to be you could route the call to Gizmo5 VOIP account and then connect directly to it over the internet. Now Google bought Gizmo5 as well (though my VOIP phone still works with it).