Tantus Networks announced their intent to roll out 500 free (to venue owners and users) Wi-Fi hotspots in the Chicago are. (See press release). This one will be interesting to watch. Local ad inventory is typically hard to sell due to the cost of the sale as well as the expectations of local advertisers – many of which will expect pay-for-performance models as they will not be satisfied with the placement short of a phone call.
It's one thing to be Google which can integrate local maps and information into their service offering (e.g. free Wi-Fi in San Francisco), and it's another to be selling advertising on splash pages. It will also be hard in the near term to operate a Wi-Fi network with a single stream of revenue (i.e., advertising). That said, most consumers are not willing to pay for public Wi-Fi so ad-supported networks will emerge in one form or another. I think it's more a question of whether ad revenues alone will be enough in the near and long term.