I have the bitcoinwireless.com and bitcoinwireless.net domains. I would be willing to transfer them to a reputable member of the community if he/she were going to do something awesome with them. I also think this is a great idea. You should offer a selection of phones, too. It would provide the opportunity of having a completely anonymous phone service, with the ability to refill without going into a retail store.
Selling Phones is something I have considered however it will require holding (depreciating) inventory so I would want to see enough demand first but I agree that it would be a great idea. You could pay for phone, SIM card, and airtime all by BTC. Only phone & SIM need physical delivery and it could be anywhere you want.
Prepaid wireless is really starting to reach parity with contracted plans.
Google is selling the Galaxy Nexus (unlocked w/ no contract usable on 200 carriers globally) direct to consumers.https://play.google.com/store/devices/details?id=galaxy_nexus_hspa
Simple Mobile (T-mobile reseller) offers unlimited plan for $40 a month.http://www.mysimplemobile.com/plans-40-unlimited.aspx