Old trick with window.scrollTo(0,1); doesn't work. And even worse, the address bar moves only a bit and gets stuck halfway out sometimes.
It is a combination of many things as I have found when researching this issue for myself. Here's the code that properly works on iOS5: (I know I'm a little late, but an answer is an answer, hopefully it can help people in the future)
<!DOCTYPE html>
<html>
<head>
<title>Hide Address Bar</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<script>
window.addEventListener("load",function() {
setTimeout(function(){
window.scrollTo(0, 0);
}, 0);
});
</script>
<style>
body { min-height: 480px; }
</style>
</head>
<body>
<h1>Content</h1>
</body>
</html>
Source: http://24ways.org/2011/raising-the-bar-on-mobile
Example: http://jsbin.com/isenax/