You might want to skip this longish introduction and go straight to the solution.

The problem
Mobile telephone manufacturers are slowly adding wifi to there telephones. Everyone seems to realize that the future is in IP networking. There is only one group strongly opposing this move towards wifi, the telephone service providers. Vodafone & Co don't like the idea of people talking to each other, without paying money for telephone services.
As the service providers are by far the biggest buyers of telephones they have managed to hold this technology back for a while, but sooner or later it will break through.
At that time the telephone market will change significantly, if it will continue to exists at all. The telephone service providers will probably be reduced to wireless internet providers, as any IP connection will do for VOIP. Their only advantage will be their existing antennae network. People will use WIFI to connect to their PC, and call from their, and only fall back the "telephone" network if their personal WIFI network is not available.

Another technology that will break through "any time now" is mesh networking. In a mesh network is a network in which hosts that can not directly talk to each other can use other hosts as intermediaries. In essence this is also the basic idea behind the internet. What makes a mesh network different is that the network is created ad-hoc, without preprogrammed routes.
The US Army uses mesh telephones in Iraq. If such a telephone can not reach the normal network, it will look for other telephones in the neighborhood, and ask those to forward the message.
The big disadvantage of mesh telephones is that networking takes a lot of power. To do it properly, all telephones must be on at all times. And the further a signal has to reach, the more power it takes. This means that the technology only works well if there are many devices around (this principle is known as Metcalfe's Law ).
In summary, for a good mesh network you want many devices at a short range, so you need only little power for transmissions.

The solution
My idea is to add a WIFI relay to every adapter. Everyone who has a telephone also owns an adapter for it. For most people this adapter is always plugged in, and almost always in the vicinity. There are so many telephones (and thus adapters) around, that any urban environment would be covered pretty well. Good coverage means that your telephone can get away with a weaker transmitter (less power usage).
Add a little mesh software, and a VOIP application, and suddenly all customers in range can call each other for free. Now that would be a good reason to buy a particular brand of telephone.

WIFI is hot. Everyone wants a wireless network. The goal is to be connected everywhere and anytime. It is possible to do that with UMTS, but you will have to pay for it, and it won't be very fast. Wifi is much nicer, but its range is severly limited; you'll have to depend on the kindness of others to provide you with open WIFI networks. Allthough everyone likes to use an open network, few people are willing to run one. Various companies try to solve this problem by creating free-for-members wifi networks, that you can only join on the condition that you share your network as well. ISP's don't like it, and don't allow it. Allthough I have not yet heard of anybody losing his internet connection because he shared it, sooner or later this will become an issue.

It doesn't have to be a problem. The cable/ADSL providers could turn this into an advantage. They should add a wifi router to every cable/adsl modem they rent out. This router should come with software that allows other customers to join the network. The router makes sure the traffic of the guest is separated from that of the host. It would even be possible to temporary raise the speed on the internet connection, so guest usage won't slow down the hosts connection. As an alternative an ISP could even pay/discount it's customers for sharing their connection.
As all the users are customers of the same provider their should be no problem with sharing the network with strangers. If a guest downloads kiddyporn it will be linked to his account, not to the host.
ISP's won't have any issue with this kind of sharing, as everyone is a customer anyway. It would even be possible to create WIFI only accounts, that solely depend on others to provide them with access points.
It doesn't have to stop there. ISP's could agree to share their networks, just like they have peering contracts for the wired internet. The only thing that is required is a strong authentication system, and there are plenty of those around to chose from.