Thirteen airports are covered in a free Wi-Fi for iPhones promotion: Boingo Wireless is reaching out to the hundreds of thousands of we early adopters with iPhones by offering free access in 13 airports. They purchased Concourse Communications last year, the leading airport Wi-Fi vendor. Boingo charges $8 for a day session or $22 for unlimited US and $39 for unlimited worldwide access. No special coupon codes are needed: Just select the "concourse" network from the list of Wi-Fi networks and connect.
Because the iPhone doesn't yet support third-party applications, using Boingo requires the entry of your user name and password on a login page each time you want to connect. This promotion should let iPhone owners test Boingo's network with less friction; ultimately, Apple will allow outside software, and a Boingo connection manager would be the first thing I'd want on mine.
The airports are Baltimore/Washington, Detroit, JFK, Kalamazoo/Battle Creek, LaGuardia, Lambert-St. Louis, Midway (Chicago), Minneapolis-St. Paul, Nashville, Newark, O'Hare, Toronto, and Will Rogers (Oklahoma City).
(An enterprising soul might recognize that the iPhone is recognized by an HTTP referrer string sent by its Safari browser.)