This morning I had what seemed like a very simple task to complete, add a tab order to a small login form in an AIR app we're building. Several hours later I still had not accomplished this. I'm the first to admit that after a year or so of managing a team and not doing much hands on development my Flex skills are a bit rusty but this was ridiculous. The login form is in a modal popup window. Turns out that was m problem. When I would hit "Tab" the focus would jump back to the fields in the main application window. To fix this I created a FocusManager instance for the login screen and then activated it when the screen loaded. Once the screen was "activated" it held the focus while I tabbed through the form fields on that screen. Happy day. The FocusManager snippet is below, it's pretty simple, only two lines.
//grab focus for the popup window so tabIndex works.
var fm:FocusManager = new FocusManager(this);
fm.activate();