Until we get boxes that are multi-network there is a long, long way to go with this technology......
No-one wants to have to buy a different box for every network - and I suspect the tech to do this already exists.
You could even be presented with a web interface where you can turn the broadcast of different network providers on and off.
For instance - I would want Three and Vodafone - so I should be able to log in to the box, uncheck T-Mobile and Orange and that's it. If a friend comes by who uses one of the networks I am not currently broadcasting, I can log in to the box, enable the networks and they would start working immediately.
The thing is - it's not even rocket science - every one of these femtocells uses a VPN to connect back to the origin network, having multiple VPN's open - isn't really that much of a network overhead - the majority of the time the only traffic on the VPN will be management and keep-alive.