I suspect what Scott suggests is true, that it's just a syntactic shorthand for interacting with any of the published keyed values or commands of a given scriptable application.

So, essentially Apple is providing a proxy object ("iTunes" in the cited example) that probably implements the "valueForKey:" etc methods and creates and compiles an AppleScript to achieve the desired property set, get, or command invocation. Plus, since Apple's in charge, they can tweak for performance and everybody wins.

Pretty cool, if simple, idea.