[Nooob]

я думаю имелось ввиду

	var mtrx:Matrix = grid.transform.matrix;
	mtrx.invert();
	var pnt:Point = mtrx.transformPoint(new Point(mouseX, mouseY));

или

	var mtrx:Matrix = new Matrix();
	mtrx.rotate(Math.PI/4);
	mtrx.scale(1, 0.8);
	mtrx.invert();
	var pnt:Point = mtrx.transformPoint(new Point(mouseX, mouseY));