On my N!, I have tried Truphone and a couple of SIP programs. None of these has given acceptable call quality - maybe two calls out of 30 attempts allowed me to have a conversation. I Compare that to TMO's UMA, which is nearly perfect. The technology is the same, so I expected more. [plus, UMA from TMO has the huge benefit of giving me VOIP on a single phone number.]
I was very hopeful about Google Voice being the transforming element to the non-carrier-dependent business model (just a data pipe), but that doesn't seem to be the way they want to go. At least not yet.
My family plan with TMO has unlimited Hotspot (a/k/a UMA) for all phones at just $9.99 per month. Even when I moved my phone to the N1, I kept the Hotspot feature, so theoretically I would have free calling if the N1 ever supported native UMA.