I know that HTML/CSS/JS can be used to write a desktop app for Mac or PC using Adobe Air. Can the same HTML/CSS/JS code base also be used to compile the project via Adobe Air to run on mobile devices (iPhone specifically)? Or, must one use Adobe ActionScript? I seem to haven come up with conflicting answers to this when googling for it.
No, you cannot use the exact same HTML/CS/JS code base for mobile apps in AIR -- the root file has to be a SWF file. Possibly you could build a StageWebView wrapper that displayed your HTML content, but that is only the case if you don't access AIR APIs in your application (well, there are probably hackish ways to communicate between the wrapper and the HTML content). It might be better to look at something like PhoneGap, which is designed for this purpose.