Recently celebrated 1000 passes sold and I've also read that he believes they will be profitable if they make $1/router/month. At 80,000+ routers that's not a bad wad of cash for a shoelace company. Also, everyone is talking about the partnership with Skype as a way for Skype users to have wifi mobile phones. That's an interesting idea, but my money is that their partnership with Skype is also about using FON routers as super nodes (see Robert Cringely's article on the problems with Skype super nodes). Also, Google funded FON without much clear synergy for Google. At the time it started hiring and funding Mozilla Firefox referrals, people took a little while to understand that beyond just "being nice" this secured Google as default search engine in Firefox and that default search engine is easily worth far more than a $1/referral for the search engine ad revenue.
So, beyond the partnerships bringing in money FON is relying on two sources of revenue: Aliens and roaming Bills
How the Digerati Can Game the FON Model
Now, I haven't tried this yet, but here's where I see the way to "game" the FON model. Basically, if you are like the majority of the FON users you live in a place where you are unlikely to get many Aliens visiting your house. So, there is little to no motivation for you to be a Bill. Given that, you sign up as a Linus so that you can roam for free. Now, if you eventually do have a place where you might make some money from Aliens then you register as a Bill. You make money from this location and yet still roam for free. While this is a bit of a weakness of the business model, the reality is that there aren't that many people who will take advantage of it.
How "My Mom" Can Game the FON model
Use bugmenot or a similar service to sign in using a Linus' username/password. This takes a far lower level of savvy and could be easier to do.
Why Most People Won't Game FON
Much like MP3 downloads - if the goods are reasonably priced then people won't resort to illegal downloads just to get the product. With FON, the prices are already very very good and I imagine that as the service expands they'll just keep getting better and better. As long as FON stays cheap people won't both to find a way around the service and everything will be fine.