Short, thoughtful recap of a lot of the memes around wifi vs 3g by the old man of the seachange, Nicholas Negroponte:
“In the future, each Wi-Fi system will also act like a small router, relaying to its nearest neighbors. Messages can hop peer-to-peer, leaping from lily to lily like frogs the stems are not required. You have a broadband telecommunications system, built by the people, for the people. Carriers are aware of this, but they discount it because they do not feel there will be sufficient coverage. They are wrong.”
» WiReD: Nicholas Negroponte: Being Wireless
[via warchalking.org]
Petafloptimism 
Nice idea.
But.
Security.
Unless my WiFi AP is outside my firewall, I’ve now got people and scary things roaming inside my network. No thanks !
Nice idea.
But.
Scalability- a dense network of APs using a small amount of spectrum could well lead to interference problems once wifi hits critical mass, resulting in slower connections. Commercial wifi operators using higher power APs placed high on buildings to provide wide area coverage will just make things worse. See http://www.smartmobs.com/archives/000180.html for a thread on spectrum limitations.