Rich raises an interesting point and certainly visible light is a promising emerging technology. However, from a mass deployment point of view, it appears to be quite a few years away, while WiFi presents an existing deployed technology base that operators should (and do) take advantage of today. So, naturally, the present focus is on WiFi. But, to me, the big point here is that cellular operators are moving away from exclusive reliance on licensed spectrum and towards using whatever access technology makes sense in a particular location. And once they figure out how to incorporate one non-cellular technology into their networks, adding others gets that much easier. So WiFi is really paving the way for whatever will come next – be it millimeter-wave, light-based or something completely different.
I understand Rich's concerns about potential radiation damage to biological tissue but it seems to me that a long term solution would be to dramatically reduce the required transmit power of the handheld device.