//0. translate the center of the DisplayObject to 0,0 (or an arbitrary offset)
//1. then rotate it 
//2. and then translate it back.

//using Matrix: 
public function centeredRotation(degrees:Number):void {
    if (this.rotation == degrees) { return; }
    var matrix:Matrix = this.transform.matrix;
    var rect:Rectangle = this.getBounds(this.parent);
    matrix.translate(-(rect.left + (rect.width*0.5)), -(rect.top + (rect.height*0.5)));
    matrix.rotate((degrees / 180) * Math.PI);
    matrix.translate(rect.left + (rect.width*0.5), rect.top + (rect.height*0.5));
    this.transform.matrix = matrix;
    this.rotation = Math.round(this.rotation);
}

//without Matrix
private function centeredRotation(degrees:Number):void{
	if (degrees == this.rotation){return;}
	var bounds:Rectangle = this.getBounds(this.parent);
	var center:Point = new Point(bounds.x + bounds.width*0.5, bounds.y + bounds.height*0.5);
	this.rotation = degrees;
	bounds = this.getBounds(this.parent);
	var newCenter:Point = new Point(bounds.x + bounds.width*0.5, bounds.y + bounds.height*0.5);
	this.x += center.x - newCenter.x;
	this.y += center.y - newCenter.y;
}