I believe you have to make use of Point conversions.
var rect:Rectangle = new Rectangle();
for (var i:int = 0; i < textField.length; i++){
rect = textField.getCharBoundaries(i);
var globalTopLeft:Point = textField.localToGlobal(rect.topLeft);
var globalBottomRight:Point = textField.localToGlobal(rect.bottomRight);
var containerTopLeft:Point = container.globalToLocal(globalTopLeft);
var containerBottomRight:Point = container.globalToLocal(globalBottomRight);
rect = new Rectangle(containerTopLeft.x,containerTopLeft.y,containerBottomRight.x-containerTopLeft.x,containerBottomRight.y-containerTopLeft.y)
}