Boingo offers ads-for-access for iPhone, iPod touch users in 28 airports: If you're traveling in the US, Canada, or the UK through one of the 28 airports operated by Boingo's Concourse division, you can trade 15 seconds of your life for 15 minutes of free Wi-Fi. The company has tested this previously, and has now rolled the deal out.
The service is enabled by JiWire, which has gradually transitioned itself from a site that developed a hotspot directory supplemented by editorial coverage and how-to's on wireless data, to one that's now hotspot directory plus hotspot advertising. The transition is interesting, as it reflects what I've seen on Wi-Fi Networking News: Wi-Fi is easier to use, as is cell data; costs for equipment is lower or you don't need to make a choice about equipment; and usage is up so far at hotspots that there's an audience there for commercial-based access.
MetroFi has famously declared free access to metro-scale services paid for by advertising to be unworkable; that may be so, given that they were the biggest proponent of it for a few years, and no other company followed them into that approach. However, metro-scale ad-supported Wi-Fi, in which residential and roaming users alike looked at banners and commercials in exchange for servcie is a far cry from the focused hotspot advertising market.
Hotspot ads involve a very open exchange between surfer and service, and JiWire pushes the watch-for-access model quite heavily. What's saving a few bucks worth to you? 15 seconds? 30 seconds? If so, we have a deal for you, they say, that also works for the advertiser and the service provider (and JiWire). It's not subtle; you have to watch the ad to gain access. But it seems like a reasonable exchange, with two hours' access up to a full day running $4 to $12 in the U.S. at paid locations. (Of course, I subscribe to Boingo Wireless's roaming service now, so I can bypass the ads in favor of paying $22 per month for unlimited usage, too. That's part of that tradeoff.)
(Disclosure: I own a very small number of share in JiWire as part of my early working relationship with them.)