TechCrunch has a post on Peep Wireless and their CES announcement of a new P2P wireless voice call system. I think going for a voice network using wifi is an admirable goal, but will no doubt have challenges. That said, there is no way to get things like this going without giving it a try.
You can not account for human behavior when it comes to stuff like this. I remember in the early 90's when people thought that multi-player internet gaming would never work because of the latency of the net. Well, what they did not count on was that folks would simply stay away from game servers that were to laggy. Same thing here, if you have queues from the UI that let you know when you can/cant use the P2P voice, you'll adjust your behavior if there is a significant payoff to using the system. There have to be enough goodies locked up in a system like this to hang in with it when its performance degrades.
I would like to see a small step before going full voice, moving short messages, images between phones in a store & forward mode, where each client/server device accepts files and forwards some of their own. Reminds me of Epizoochory(really? yep, really), or the dispersal of seeds by animals. Imagine all the interesting stuff that would drift around the planet(just encrypt what you want private) when passed by proximity as phones make and break connections in freeway traffic, at the mall, etc. What if your local venue could not only check-in to you, but could tag you with a file that spreads to your friends just by being near them?