[twitter-bootstrap] Add icon to submit button in twitter bootstrap 2

I want to use the twitter bootstrap icons on my form input submit buttons.

The examples on http://twitter.github.com/bootstrap/base-css.html#icons mainly show styled hyperlinks.

The closest I've come is getting the icon displayed next to the button, but not inside.

<div class="input-prepend">
   <span class="add-on"><i class="icon-user icon-white"></i></span>
   <input type="submit" class="btn-primary" value="Login" >
</div>