Fon (the wifi exchange company) just got some major backing. It will be very interesting to see if it takes off.
I can certainly see it working, but it may be difficult for Fon to penetrate locations such as airports that may have exclusive contracts with some providers (such as tmobile) or will charge very high rates to lease their airspace.
As it stands today, wireless internet access is much too expensive for the average user. Whether it be Verizon EVDO or T-Mobile wifi, the costs are just too much unless your company is paying for it.