Would that work? Wi-Fi only phone service? Bandwidth.com has launched Republic Wireless (about 19 months ago I guess). It is an MVNO of Sprint but the cell network is the back-up service. Primarily, the VoIP runs over any available Wi-Fi network to keep costs and quality down. ARS has a review here. ARS isn't happy that only one phone is available.

Not for nothing, if the cell phone you currently use has Wi-Fi capability, there are number of apps that allow you to make VoIP calls from yoru cell phone utilizing an available cell network (2.5G/3G/4G) or a Wi-Fi network. This isn't anything new. It's just another way to package an MVNO and arbitrage the service.