居中水平UL

<div id=\"sitenavdiv\">
    <ul id=\"sitenav\">
        <li class=\"about\"><a href=\"#\" title=\"\"><img src=\"images/about-link.png\" alt=\"about\"></a></li>
        <li class=\"portfolio\"><a href=\"#\" title=\"\"><img src=\"images/portfolio-link.png\" alt=\"portfolio\"></a></li>
        <li class=\"contact\"><a href=\"#\" title=\"\"><img src=\"images/contact-link.png\" alt=\"contact\"></a></li>
    </ul>
</div>

#sitenavdiv {padding-top: 30px; width: 620px; position: relative; float: left; text-align:center;}

ul#sitenav {width: 231; margin: 0 auto; display: inline;}
ul#sitenav li.about{width: 64px; height: 31px; display: block; float: left; overflow: hidden;}
ul#sitenav li.about a{width: 64px; height: 31px; display: block;}
ul#sitenav li.about a:hover{width: 64px; height: 31px; display: block; margin-top:-31px;}

ul#sitenav li.portfolio{width: 88px; height: 31px; display: block; float: left; overflow: hidden}
ul#sitenav li.portfolio a{width: 88px; height: 31px; display: block;}
ul#sitenav li.portfolio a:hover{width: 88px; height: 31px; display: block; margin-top:-31px;}

ul#sitenav li.contact{width: 79px; height: 31px; display: block; float: left; overflow: hidden}
ul#sitenav li.contact a{width: 79px; height: 31px; display: block;}
ul#sitenav li.contact a:hover{width: 79px; height: 31px; display: block; margin-top:-31px;}