AIR is provided for free, of course, as Adobe wants to encourage development of Flash 10 products. The flexibility of running a program locally, but having the constructs of web development available, give web developers a much different way of interacting with a computer and ultimately with the user, and that is exactly what Adobe tries to accomplish with AIR. Despite being less than a year old, the platform has been a success for Adobe, who has several big name companies developing applications under it, such as eBay and AOL, and has drawn attention from independent developers as well.
AIR moves in a different direction that cloud computing does, though it certainly isn't a complete opposite, as the AIR environment does provide a simple way to work with online content as well, while still retaining the ability to work with offline content. AIR 1.5 is downloadable on all three platforms, for free, from Adobe's site.