Mine

this.onMouseMove = function() {
	constrainedMove(bg_mc, 1, -1);
};
function constrainedMove(target:MovieClip, speed:Number, dir:Number) {
	var mousePercent:Number = _xmouse/Stage.width;
	var mSpeed:Number;
	if (dir == 1) {
		mSpeed = 1-mousePercent;
	} else {
		mSpeed = mousePercent;
	}
	target.destX = Math.round(-((target._width-Stage.width)*mSpeed));
	target.onEnterFrame = function() {
		if (target._x == target.destX) {
			delete target.onEnterFrame;
		}
if (target._x>target.destX) {
			target._x -= (target._x-target.destX)*(speed/100);
		}
if (target._x<target.destX) {
			target._x += (target.destX-target._x)*(speed/100);
		}
	};
}