Rory : AIR is flash for mobile. In fact, it's the (extended) Flash VM, as a binary, that work on desktop and mobile. You then write all your game/app in actionscript (flash) in the regular Flash editor, and export it to AIR.
The exporter is going to bundle the VM with your binary, for your desired target.
Edit : in fact, for lots of tasks, Actionscript will beat JS/HTML5, because of access to native features allowed to the VM, that aren't allowed for the Javascript runtime...
Pode
Did you know about Tizen and Firefox OS which use HTML5 as native language?