Our favorite WiFi geek, Rob Flickenger, has been experimenting with long-range WiFi at distances of 5 miles and simulated distances of over 20. Problems? None. Although he was using an expensive spectrum analyzer to help focus the two dishes that focused the signal.
This kind of testing has a large degree of importance for vernacular networks, set up in communities that might want to offer free line-of-sight WiFi, or for companies that want to broadcast WiFi over larger distances. However, the FCC limitations in power remain, and networks running in the 2.4 GHz bandwidth must be designed and set up specifically to avoid interferring with other systems operating in the same areas.