I'm not sure GV from your Droid is VoIP...I think it still calls your cell back and would thus be using your cell minutes. This is how it has been, and I'm not sure this new feature changes that (yet).
I have a work around with Sipdroid and still haven't perfected it. Haven't really tried directly from Gmail yet, but the workaround is a little staticy and hard to hear - I probably need to incorporate a PBX address, but that might create lag.
I guess you can try an outgoing call thru GV and see if it shows up in your minutes. But the flipside is a call to your GV number, where does it go? You have no "softphone" on your phone to install (though incoming calls to something like sipgate is free, so you could configure sipdroid for that). If GV forwards to your cell, it's going to use your minutes.
The free incoming has never really been a problem, it's the free outgoing to get totally free VoIP that is tricky. I'm sure Google will eventually have a VoIP service for the Droid, but it sounds like it isn't going to be free (my guess is 1-2 cents per minute for US & Canada).