I have seen long range wifi antenna's with decent range (around 5-10 miles) for boats sailing along the coast. You would need very good directional antennas and modified transmitters and receivers and there probably can't be any obstacles in the way like buildings..
According to wikipedia it is possible. Even longer ranges like 200 miles with satelite dishes for antennas on both ends.
http://en.wikipedia.org/wiki/Long-range_Wi-FiAnyway that is an interesting project.
Let us know if you can make it work.