Coffee shops all across America struggle with the Wi-Fi question. Giving people a place to sit and work that's out of their house (or in my case today, hotel room) is a great way to drum up business during nonpeak times. But it slows turnover at peak business moments and can cause you to lose customers.
The Coffee Bar in San Francisco has an effort to solve this that I haven't seen before. There's a set of tables that are marked out with cards as "laptop free" specifically during peak lunch service time but not otherwise. It's a good idea in principle, especially if you think you can count on your customers to be motivated by norms.