The German FON blog reminds the users to use only official FON software. They quote point 6.7 of the general conditions.
The German version:
Um eine korrekte Funktion der FON Hotspots zu gewährleisten, müssen die Linuse und Bills ausschließlich offizielle Versionen der FON Software einsetzen.The English version:
In order to guarantee the correct functioning of the FON Hotspots, the Linuses and Bills shall exclusively use the official versions of the FON Software.
It is interesting that the German version says "müssen" (engl. to have to) and the English version says "shall". I am not a lawyer, but it looks strange to me. Anyone (especially lawyers) knows how to interpret such terms please leave a comment.
The following is fully neutral: Please read the comments and answers of Peter of the blog. Very interesting... Sorry, but this is in German. But the non-German readers can try Google translate. (Bear in mind that the quoted part of the condition is also translated from the German version.)
Furthermore the German FON blog is moderated, this means submitted comments need further approval. Stefan offered them information about a security flaw with the firewall rules when using multiple IP subnets. His comment is still not published yet...