it does not work in general, but it is a starting point:

//
// only test code...
// does only work in a few cases...
// g.s
//

var obj = document.selectedItems[0];

var center = obj.bounds.center;
var corner1 = obj.curves[0].point1;
var corner2 = obj.curves[1].point1;

var refPoint = new Point(corner2.x,corner1.y);

print(corner1);
print(corner2);

document.createCircle(corner1, 3);
document.createCircle(corner2, 3);
document.createCircle(refPoint, 3);

var ak = refPoint.getDistance(corner2);
var gk = refPoint.getDistance(corner1);

var tan = gk / ak;
var angle = Math.atan(tan);

print(angle);

var refRect = document.createRectangle(Rectangle(0, 0, 50, 50));
refRect.rotate(angle,refRect.bounds.center);

look at these for some explanation: http://img518.imageshack.us/img518/9151/rotatell2.gif

i think there is a better way to go, but this was my first idea how to solve the problem.
i'd like to see other solutions...

greetings
g.s

Thanks alot for your reply, this is the solution I've come up with, and it is working well for me, in every angle.

sel = activeDocument.getMatchingItems(Path,{selected:true});

function getPoint(i){
	// get the point of the rectangle that was originally the upper left one
	return(sel[i].segments[2].point);
}


function drawBaseLine(i){
	// draw a new path displaying the baseline
	baseline = new Path();
	baseline.segments.add(new Point(sel[i].bounds.center.x-75,sel[i].bounds.center.y));
	baseline.segments.add(new Point(sel[i].bounds.center.x+75,sel[i].bounds.center.y));
}

function graden(){
	// calculate the angle in which the corner point is compared
	// to the centerpoint of the square.
		
	xDiff = Math.abs(sel[0].bounds.center.x - getPoint(0).x);
	yDiff = Math.abs(sel[0].bounds.center.y - getPoint(0).y);
				
	var thisTan = yDiff / xDiff;
	var angle = Math.atan(thisTan);
	
	graden = angle * (180 / Math.PI);
	
	return(graden);
}

// draw a circle to indicate which of the four points
// was originally in the upper left corner
activeDocument.createCircle(getPoint(0), 15);
drawBaseLine(0);


if (getPoint(0).x < sel[0].bounds.center.x){
	
	// figure out in which area the corner point is
	// compared to the center point of the square, and
	// add the appropriate amount of graden to the
	// result of the tangens to get the value of the
	// rotation.
	
	if (getPoint(0).y > sel[0].bounds.center.y){
		print ("upper left corner: ");
		result = graden()-45;
		if (result < 0){
			result += 360;
		}
		print ("rotation: " + Math.round(result) + " graden");

	}
	else {
		print ("lower left corner");
		result = (90 - graden()) + 225;
		print ("rotation: " + Math.round(result) + " graden");
	}
}

else if (getPoint(0).x > sel[0].bounds.center.x){
	
	if (getPoint(0).y > sel[0].bounds.center.y){
		print ("upper right corner");
		result = (90 - graden()) + 45;
		print ("rotation: " + Math.round(result) + " graden");
			
	}
	else {
		print ("lower right corner");
		result = graden()+135;
		print ("rotation: " + Math.round(result) + " graden");
	}
}

And an image of some action, click link for full image:
http://www.thijsgadiot.nl/calcAngle.png