Problem With Dropdowns in WordPress 3.0 Menu

I’m working on a theme at the moment and have run into trouble with the WordPress 3.0 menu – more specifically the submenu/dropdowns. The submenus are not functioning as they should – they are always appearing under their parents as opposed to only appearing when hovering over their parent. Here is my code.

This from my themes functions.php file:

// add/register WordPress 3.0 menu

add_action( 'init', 'register_my_menu' );

function register_my_menu() {
	register_nav_menu( 'primary-menu', __( 'Primary Menu' ) );
}

This is in my header.php file:

<div id="top-menu">

<?php wp_nav_menu( array( 'theme_location' => 'primary-menu' ) ); ?>

</div>

And finally, here is my CSS:

#top-menu {
	font-family: 'Droid Sans', serif;
	font-size: 14px;
	font-weight: bold;
	color: #413F40;
	padding-top: 13px;
}

#top-menu a:link, #top-menu a:active, #top-menu a:visited {
	line-height: 27px;
	padding: 0 8px 1px 8px;
	display: block;
	color: #413F40;
	text-decoration: none;
}

#top-menu a:hover {
	line-height: 27px;
	padding: 0 8px 1px 8px;
	display: block;
	color: #FFF;
	text-decoration: none;

}

#top-menu li {
	list-style: none;
	float: left;
	margin-right: 20px;
	height: 27px;
}

#top-menu li:hover {
	background: url(images/top-menu.png);
	list-style: none;
	float: left;
	margin-right: 20px;
	color: #FFF;
	height: 27px;
}

#top-menu ul {
	padding-left: 0;
}

Here is a screenshot of my menu area:

http://cl.ly/6Jdt

As you can see, “Contact” has one submenu item that is simply appearing underneath it permanently. Have I missed something? Thanks!