Yup, that worked great. Hope this helps someone searching...
Code:
Stage.scaleMode = "noScale";
Stage.align = "T";
var catchStage:Object = new Object();
Stage.addListener(catchStage);
catchStage.onResize = function() {
if (Stage.height/Stage.width > bg_mc._height/bg_mc._width) {
img_prop = bg_mc._width/bg_mc._height;
bg_mc._height = Stage.height;
bg_mc._width = Stage.height*img_prop;
bg_mc._y = 0;
bg_mc._x = 0;
} else {
img_prop = bg_mc._height/bg_mc._width;
bg_mc._width = Stage.width;
bg_mc._height = Stage.width*img_prop;
bg_mc._y = 0;
bg_mc._x = 0;
}
}