[i.o.]

Вот из справочки пример:

var loadListener:Object = new Object();
 
loadListener.onLoadComplete = function(target_mc:MovieClip, httpStatus:Number):Void {
    trace(">> loadListener.onLoadComplete()");
    trace(">> =============================");
    trace(">> target_mc._width: " + target_mc._width);    // 0
    trace(">> httpStatus: " + httpStatus);
}
 
loadListener.onLoadInit = function(target_mc:MovieClip):Void {
    trace(">> loadListener.onLoadInit()");
    trace(">> =============================");
    trace(">> target_mc._width: " + target_mc._width);    // 315
}
 
var mcLoader:MovieClipLoader = new MovieClipLoader();
mcLoader.addListener(loadListener);
 
var mc:MovieClip = this.createEmptyMovieClip("mc", this.getNextHighestDepth());
mcLoader.loadClip("http://www.w3.org/Icons/w3c_main.png", mc);

Т.е. нужно подписаться еще и на onLoadInit