To jQuery Mobile this is a regular link to jQuery Mobile inner page, or to put it in different words, another page inside a same HTML. But why would someone call his page like that is beyond me.
One more thing to notice, page with that id would fail to raise any jQuery Mobile page event.
It can be tested here: http://jsfiddle.net/Gajotres/3278E/
HTML :
<!DOCTYPE html>
<html>
<head>
<title>jQM Complex Demo</title>
<meta name="viewport" content="width=device-width; initial-scale=1.0; maximum-scale=1.0; minimum-scale=1.0; user-scalable=no; target-densityDpi=device-dpi"/>
<link rel="stylesheet" href="http://code.jquery.com/mobile/1.2.0/jquery.mobile-1.2.0.min.css" />
<script src="http://code.jquery.com/mobile/1.2.0/jquery.mobile-1.2.0.min.js"></script>
</head>
<body>
<div data-role="page" id="index">
<div data-theme="a" data-role="header">
<h3>
First Page
</h3>
</div>
<div data-role="content">
<a href="#listing.php" data-role="button">Another Page</a>
</div>
<div data-theme="a" data-role="footer" data-position="fixed">
</div>
</div>
<div data-role="page" id="listing.php">
<div data-theme="a" data-role="header">
<h3>
Second Page
</h3>
<a href="#index" class="ui-btn-left">Back</a>
</div>
<div data-role="content">
</div>
<div data-theme="a" data-role="footer" data-position="fixed">
</div>
</div>
</body>
</html>