Developing low-cost WiFi hotspots with a Bitcoin time charge is a worthy project. Some people do not have their own dedicated internet provider. Imagine if they could access one of these from home, provided by an entrepreneurial neighbor. Multiply that by hundreds. "Make big money selling internet access to your neighbors!" It remains to be seen if Raspberry Pi is optimal for it, but it makes sense to start with a low-cost method first. Many of the innovations will be applicable to other hardware configurations as well.
I also think this is a great project (in particular if you plan to not only sell at about the parts' costs but also to release the software under a free license!), but I doubt you could earn a lot of money. Isn't the trend rather going towards free WiFi by cities and such? I currently pay about 20 EUR per month for my home broadband connection, so I think for someone to actually "buy" internet access from his/her neighbour instead of getting their own it would have to be less than 10 EUR per month or so, and probably even still less.
Not to mention that my contract would surely be terminated immediately and I possibly even sued by my ISP if they found out that I sold my home connection to strangers - I didn't read the details, but this is guaranteed to be prohibited by the ToS of almost any service I can imagine. (At least for private home owners.)
