The wireless backbone provider Towerstream will flip on a dense Manhattan Wi-Fi network: Towerstream built a wireless network in the skyline, paying for prime locations on the top of buildings to point high-speed service at line-of-sight locations where conventional wired or even fiber broadband wasn't available, would take too long, or wasn't competitive or reliable enough. Now it's taking aim at Wi-Fi. But it's not trying to be a metro-scale Wi-Fi operator. That would be foolish. Rather, Towerstream is building out a dense Wi-Fi zone, described by BusinessWeek as seven square miles of Manhattan. The firm is deploying 1,000 routers, and the backhaul is clearly its own building-top network. Being able to leverage its own backhaul is a distinct financial advantage, as it already has a business model that works for the point-to-multi-point service it offers today. This is icing on the cake.…