In two applications that I'm writing, the content inside a SWFLoader does not participate in the tab navigation. The textfields in the loaded SWF are focusable and I can even detect Tab keystrokes in them, but there is no tab navigation.
The setup is very simple - an application (in one case mx, another case spark) which contains a SWFLoader.
I've seen one thread in a dev forum where the solution was to implement a focusmanager. I'm hoping for something simpler...