test in jquery if element is at top of screen
<p>I have a div that is positioned about 100px from the top of the browser window.  When the user scrolls down, I want the div to stay where it is UNTIL it reaches the top of the screen.  Then, I'll change some css in jquery to make the position to change to fixed and the margin-top to 0.  How can I test in jquery if this div is at the top of the screen?</p>
