As you know that to get connected to a Wi-Fi network you need to be in close proximity of the access point and this coverage area hasn't increased much even though we've moved from b to g and well on the course to 802.11n; but the case with the cell phone is that we are constantly moving from one cell to another and one access point would be virtually impossible to cover an area equivalent of one normal cell that a modern day cell-tower covers. Then I think the question of hand-off will also impede the Wi-Fi phones to attain ubiquity; which I am suppose would be quite difficult or is it even possible? I am not too sure about it! Whatever may be the reason, I still believe it is good to have a choice of Wi-Fi on a phone on top of the other connectivity options; as such when we are in close proximity of our trusted network, it becomes more feasible to get connected.