This should work:
$('div:not(#myDiv)').hide(); // hide everything that isn't #myDiv
$('#myDiv').appendTo('body'); // move #myDiv up to the body
Update:
If you want to hide EVERYTHING that, not just div elements, use this instead:
$('body > :not(#myDiv)').hide(); //hide all nodes directly under the body
$('#myDiv').appendTo('body');
Probably simpler is to wrap the entire “hideable” part of the page in a big container element, and hide that directly though.
Like so:
<body>
<div id="contents">
<!-- a lot of other stuff here -->
<div id="myDiv>
</div>
</div>
</body>
Then you can just do this, which is cleaner and faster:
$('#contents').hide();
$('#myDiv').appendTo('body');