My gut instinct tells me that Adobe AIR won’t be mainstream despite some nice looking applications. Why? Isn’t it easier for most purposes to be in the browser, with a single set of enhancements (like plugins – how mainstream are they though) that suit your working style?
To counter that instinct, Adobe’s example is of iTunes – a web-technology enhanced stand-alone application. The critical difference with iTunes is that Apple gives you no choice over alternative routes to the iTunes shop. I guess Air might allow people to be Apple-like for less. You can see eBay say: “our Air application is the best route for power sellers”, for example.
The ability of a rich application to knit together web scale (such as S3 storage, web service lookup, Wikipedia) in a highly-functional local interface does sound valuable, and willsuit high volume application users – like eBay power sellers.
The data will still be stored securely on the web, accessible anywhere with a browser (and I mean out of home and office as well as on mobiles, etc).
The issue must be how effective the security of the platform will be: if you can install a dodgy application that has full access to my machine and the web, you can do what you like!