format pages list within list returned b PHP

Hi,
I have this code in my header php getting me the list of pages

<div class="center-column">
            <ul id ="menu">
  <?php wp_list_pages('sort_column=menu_order&title_li='); ?>
</ul>
			<div class="clearboth"><!-- --></div>
		</div>

how do I set up my css to handle the returned items.
At the moment it displays like this

Main 1 Main 2 Main 3
sub page 1 sub page 2

I want it to look like this

Main 1 Main 2 Main 3
sub page 1
sub page 2

Problem is a list within a list, I am not sure how to fix it.
my ul CSS below

ul#menu {margin:0; padding:0;}
ul#menu li {font-size:0.916em; float:left; margin-right:1.819em; padding:0; display:block; list-style:none;}
ul#menu li a:link, ul#menu li a:visited {display:block; padding:7px 0 10px; line-height:1em; width:9.08em; color:#6a797d; font-weight:bold; text-decoration:none; border-top:3px solid #fff; text-transform:lowercase;}
ul#menu li a:hover {border-top:3px solid #ff8a00;}
ul#menu li a span {font-size:0.833em; color:#d1d9dc; text-transform:lowercase;}
ul#menu li a.selected {border-top:3px solid #6a797d;}
ul#menu .last {margin-right:0;}

thanks for any help