The Fon blurb says
In reference to support.fon.com/entries/20106041-How-do-Foneras-work-:The Fonera routers use your existing connection and send out a public and private wireless signal. It is the public signal that makes your Fonera a Fon Spot. You will use the public signal of other Foneras to connect when you are travelling. The private signal is strictly for you, your family, and friends that live in your residence since it is encrypted.
But how does the security on that work? What stops people connecting over the "public signal" from being access your LAN once they have a connection, via the Fon WAP, to your router?