The rapid growth of WiFi over the past two years reveals the willingness of the Wireless Internet Service Providers (WISPs) to develop this technology and the interest of the users in this new service. However, the lack of unified roaming slows down the deployment of this kind of networks. In this paper, we present a simple solution that allows a mobile node to connect to a hot spot managed by a WISP in a secure way while preserving its anonymity. We use a reputation-based system to discourage the WISPs from providing a bad quality of service to the mobile nodes. We will show that all the parties are motivated to behave correctly, that our solution thwarts rational attacks, that it is possible to detect malicious attacks and, in some case, identify the attackers.