Why go to all that trouble, when any user could become a vendor for any of those prepaid services and put up a website that allowed users to buy minute cards & top up cards for bitcoin.  I use Virgin Mobile, and the local vendor that I usually buy topup cards from doesn't even use actual cards anymore, but generates a new top up code that is printed onto the receipt.  I'm sure that coding this into a website would be trivial for any code monkey worth his 'leet thinkgeek t-shirts.