I have been digging for custom Flex preloaders and they all seem to rely on the same template:
An SWC is created with Flash CS5 and then used by Flash Builder using the "preloader" application property.

I don t own Flash CS, and it feels that Flash builder should be able to do the trick.
I created a Library Project in Flash Builder with the following bare bones code:

package loader  
{
    import flash.display.DisplayObject;
    import flash.events.Event;
    import flash.utils.getTimer;

    import mx.events.RSLEvent;
    import mx.preloaders.DownloadProgressBar;
    import mx.preloaders.SparkDownloadProgressBar;      

    public class Preloader extends SparkDownloadProgressBar
    {

        [Embed(source="loaderlogo.png")] public var logoClass:Class;

        private var _displayStartCount:uint = 0; 
        private var _initProgressCount:uint = 0;
        private var _downloadComplete:Boolean = false;
        private var _showingDisplay:Boolean = false;
        private var _startTime:int;
        // private var preloaderDisplay:PreloaderDisplay;
        private var rslBaseText:String = "loading: ";


        public function Preloader()
        {
            super();
        }

        /**
         *  Event listener for the <code>FlexEvent.INIT_COMPLETE</code> event.
         *  NOTE: This event can be commented out to stop preloader from completing during testing
         */
        override protected function initCompleteHandler(event:Event):void
        {
            dispatchEvent(new Event(Event.COMPLETE)); 
        }

        /**
         *  Creates the subcomponents of the display.
         */
        override protected function createChildren():void
        {    
            var img:DisplayObject = new logoClass();
            img.x = Math.round( ( stageWidth - img.width) / 2);
            img.y = Math.round( ( stageHeight - img.height) / 2);
            addChild( img);

            var dpb:DownloadProgressBar = new DownloadProgressBar();
            dpb.x = img.x + 100;
            dpb.y = img.x + 100;
            dpb.width = 170;
            dpb.height = 20;
            addChild( dpb);
        }

        /**
         * Event listener for the <code>RSLEvent.RSL_PROGRESS</code> event. 
         **/
        override protected function rslProgressHandler(evt:RSLEvent):void {
            if (evt.rslIndex && evt.rslTotal) {
                //create text to track the RSLs being loaded
                rslBaseText = "loading RSL " + evt.rslIndex + " of " + evt.rslTotal + ": ";
            }
        }

        /** 
         *  indicate download progress.
         */
        override protected function setDownloadProgress(completed:Number, total:Number):void {

        }

        /** 
         *  Updates the inner portion of the download progress bar to
         *  indicate initialization progress.
         */
        override protected function setInitProgress(completed:Number, total:Number):void {
        } 


        /**
         *  Event listener for the <code>FlexEvent.INIT_PROGRESS</code> event. 
         *  This implementation updates the progress bar
         *  each time the event is dispatched. 
         */
        override protected function initProgressHandler(event:Event):void {
            var elapsedTime:int = getTimer() - _startTime;
            _initProgressCount++;

            if (!_showingDisplay &&    showDisplayForInit(elapsedTime, _initProgressCount)) {
                _displayStartCount = _initProgressCount;
                show();
                // If we are showing the progress for the first time here, we need to call setDownloadProgress() once to set the progress bar background.
                setDownloadProgress(100, 100);
            }

            if (_showingDisplay) {
                // if show() did not actually show because of SWFObject bug then we may need to set the download bar background here
                if (!_downloadComplete) {
                    setDownloadProgress(100, 100);
                }
                setInitProgress(_initProgressCount, initProgressTotal);
            }
        }

        private function show():void
        {
            // swfobject reports 0 sometimes at startup
            // if we get zero, wait and try on next attempt
            if (stageWidth == 0 && stageHeight == 0)
            {
                try
                {
                    stageWidth = stage.stageWidth;
                    stageHeight = stage.stageHeight
                }
                catch (e:Error)
                {
                    stageWidth = loaderInfo.width;
                    stageHeight = loaderInfo.height;
                }
                if (stageWidth == 0 && stageHeight == 0)
                    return;
            }

            _showingDisplay = true;
            createChildren();
        }

    }
}

For short, it s loading a logo and a progress bar
It displays a preloader, but really late in the loading process. As if it was being loaded after Flex.
Do I need to compile this in CS5 to completely avoid use of MX/Spark?