My initial impression of Adobe AIR was that it was some sort of "Flash for the desktop" deal. Developing apps in Flash didn't really appeal to me so I dismissed it (bad memories Flash development using early versions of Macromedia's tools).
Fast forward to a few weeks ago, I started experimenting with Twitter (basically publicly broadcasted instant messages). In so doing I looked at a number of desktop clients, of which it turns out there are a several that run on the Adobe AIR runtime. One of the first AIR twitter clients I tried was Jonathan Snook's Snitter. Being a Snook creation it looks great (and of course, lime green by default). It's even got a little notification area (née systray) icon for when it's sitting in the background minimised.
There's nothing that belies the fact that this is really just an AJAX-y web application without the browser. It consists of just a bunch of HTML, CSS and JS files sitting in my Program Files directory (C:\Program Files\Snitter to be exact). I had a bit of fun opening them up in a text editor and hack around with them and was pleased to see that Mr Snook is a also a jQuery fan.
It seems that AIR really picks up from where HTAs left off with the added promise of cross-platform compatibility. The Adobe AIR runtime is impressively small (10MB), fast and easy to install. (This highlights a bugbear I have with the .NET Framework these days: it really needs to go on a diet, or become less monolithic. The .NET 3.5 runtime is over 100MB in total, whereas .NET 1.x was about 20MB.)
Anyway I digress. If you want to dig into creating AIR apps in more detail, Jonathan Snook has just written seasonal overview, which goes into topics such as data storage, file system access and application packaging.