Wandering around the 3rd Arondissement in Paris (I haven't ventured further yet), there is a "Free(tm) Wifi" ("Free" is a company, it's not actually free) literally everywhere, usually with full signal bars. I don't really trust signal bars after having read that people can configure routers to blast full Tx power but that doesn't help reception, so I actually tried joining the networks from my phone - they connect fine. I don't have an account so I can't test the actual Internet (see previous post), but at least I can see the portal page.
There's also an "SFR Public Wifi" in almost, but not quite, all locations, but the signal strength is almost always less.
There has only been one place where I noticed FON availability, but its signal strength was so weak I couldn't connect to the network.
Although I haven't actually gotten to use the Internet at any of these nodes yet, the experiment is still interesting - as is wandering around with Erin, who doesn't care at all. She wants a SIM card so it "just works." (Her phone is a proper GSM unlocked iPhone, unlike my useless-in-Europe CDMA Nexus S.) Of course she does: why would you want something that doesn't just work? (We'll try to get a SIM card later, but things like that don't happen in France on a Sunday.)
For me this is an experiment in whether public shared wifi could ever be a viable option on its own. Free.fr is about as close as you can possibly get, and it really does saturate this place so entirely that I think it might be usable as an alternative to a cell phone plan, assuming you dealt with mobile IP roaming issues, bandwidth limits, etc, which ought to all be manageable, albeit difficult. But it doesn't work if you leave the city, and it seems to require a city with the density of Paris to actually pull it off. I don't know how popular SFR.fr is, but they're not available quite everywhere in town, so it would be endlessly frustrating to try to use it (eg. for maps) instead of a cell plan. The wifi is purely a second-choice network that lets you offload the cell network a bit and sometimes avoid metering. Regardless of whether you can subscribe to it without buying a cell phone plan (you can't), it wouldn't be a positive user experience.
Of course, if both providers would just auto-connect all their customers to FON, then the combined coverage would be astonishing. That would reduce Free.fr's advantage over SFR.fr, though, so why would they do that?
All this makes the concept of public wifi look rather grim. The business model isn't there (you make much more money selling cell phone plans) and the user satisfaction isn't there (nearly anyone would prefer a slow, always-on cell data plan over fast, flakey, often nonexistent wifi data).
Or maybe I'm being shortsighted and need to think bigger. Maybe it's possible to super-ultra-saturate an area, for a reasonable price, and make a business of it. I think a key component of such a plan would be the resolve to not become also a cell provider. Cell providers have a perverse incentive to make sure wifi coverage is imperfect.