<body>
<!-- All I've done is add in a .spacer class in line 6 of the CSS and
put a <div class="spacer"></div> in as a third div inside your
#bodyContainer, so now its #leftNav, #content and .spacer which
makes it wrap -->
<div id="container">
<!--header-->
<div id="headerContainer">Header</div>
<div id="bodyContainer">
<div id="leftNav">
<ul>
<li><a href=''>Admin</a>
</li>
<li><a href=''>Admin</a>
</li>
<li><a href=''>Admin</a>
</li>
<li><a href=''>Admin</a>
</li>
<li><a href=''>Admin</a>
</li>
<li><a href=''>Admin</a>
</li>
<li><a href=''>Admin</a>
</li>
<li><a href=''>Logout</a>
</li>
</ul>
</div>
<!--leftNav-->
<div id="content">
<form id="form" method="post" action="index.php">
<div>
<label for="username">Username</label>
<input type="username" name="username" value="username">
</div>
<div>
<label for="password">Password</label>
<input type="password" name="password" value="password">
</div>
<div id="submitDiv">
<input type="submit" name="submit" value="submit">
</div>
</form>
</div>
<!--content-->
<div class="spacer"></div>
</div>
<!--bodyContainer-->
<div id="footerContainer">Footer</div>
<!--footer-->
</div>
<!--container-->
</body>