You know Wi-Fi has really hit the mainstream when art students use it in their performance art pieces: Students from New York's Parsons Design and Technology have built access points into bicycles and will use them to send emails from New York subways. The architects of this idea seem a bit torn between regarding it as performance art and pointing to its utility.
There's not a very detailed techincal explanation for how this works, but it appears that the APs use cell networks for backhaul or are used as repeaters to extend signals from other hotspots.
It's kind of a cool idea for delivering Wi-Fi connections on short notice or for a temporary reason. Or, wouldn't it just be cool to have so that you could be sure of having a connection--and be able to share it with pals--anywhere?