Here’s how to use the :before pseudo selector and glyphicons for Bootstrap 2.3.2 instead of a background image on the input.

Here’s a couple of simple examples: http://jsfiddle.net/qdGZy/
<style type="text/css">
input.search-query {
padding-left:26px;
}
form.form-search {
position: relative;
}
form.form-search:before {
content:'';
display: block;
width: 14px;
height: 14px;
background-image: url(http://getbootstrap.com/2.3.2/assets/img/glyphicons-halflings.png);
background-position: -48px 0;
position: absolute;
top:8px;
left:8px;
opacity: .5;
z-index: 1000;
}
</style>
<form class="form-search form-inline">
<input type="text" class="search-query" placeholder="Search..." />
</form>
Update For Bootstrap 3.0.0

Here’s an updated fiddle for bootstrap 3.0: http://jsfiddle.net/66Ynx/