Voice over W-Fi can't make it in the mainstream unless it offers a different application than cell phones: So theorizes this ZDNet UK columnist and he has some good points. The fact that cellular networks are already ubiquitous and the majority of the population in some countries have cell phones means that voice over Wi-Fi can't be a me-too service or no one will want it. He thinks that Wi-Fi will need a killer app, presumably one that takes advantage of its high broadband capabilities. Music or video services might convince potential customers to buy into voice over Wi-Fi.
The most important unknown with voice over Wi-Fi is the role that the cellular carriers will play. If they decide they want to take advantage of Wi-Fi and start building chips into their phones, then voice over Wi-Fi will work in the mass market. But so far the operators have been very non-committal on where they stand with integrating Wi-Fi into their phones.