<div id="retangulo" style="position: absolute; background: #eef; width: 200px; height: 200px; left:200px; top: 200px;">Mova o mouse fora da rea do retngulo</div>
<div id="dot" style="position: absolute; border:1px solid #000; background: #fee; width: 20px; height: 20px;"></div>

<script type="text/javascript">
//<![CDATA[

var square = document.getElementById("retangulo"), box = document.getElementById("dot");

//http://www.jsfromhell.com/geral/event-listener
addEvent(document, "mousemove", function(e){
	var p = closestSquarePoint(e.clientX, e.clientY, square.offsetLeft, square.offsetTop, square.offsetWidth, square.offsetHeight);
	box.style.left = (p.x - box.offsetWidth / 2) + "px";
	box.style.top = (p.y - box.offsetHeight / 2) + "px";
});

//]]>
</script>