Question

This list is working great for me but the text within the <li> elements is not centering.

The <li>s must auto resize to their content.

#nav-menu {
    font-family: Verdana, Arial, Helvetica, sans-serif;
    height: 30px;
    background-image: url(../img/menu_bg.png);
    background-repeat: repeat-x;
    border-bottom: dotted thin #666666;
    border-top: dotted thin #666666;
    text-align: center;
    width: 800px;
}

#nav-menu ul {
    list-style: none;
    padding: 0;
    margin: auto 0;
}

#nav-menu li {
    float: left;
    border-right: dotted thin #666666;
    list-style: none;
    padding: 0.5em 2em 0.5em 0.75em;
}

#nav-menu li a {
    line-height: 1.5em;
    color: #333333;
    text-decoration: none;
    font-size: 12px;
    font-weight: bold;
    display: block;
}

<div id="nav-menu">
    <ul>
        <li class="current_page_item"><a href="#" title="Home">Home</a>
        <li class="current_page_item"><a href="#" title="Home">Home</a>
        <li class="current_page_item"><a href="#" title="Home">Home</a>
        <li class="current_page_item"><a href="#" title="Home">zxczczxczHome</a>
    </ul>
</div>

Answer 1

While you’re assigning unequal padding values to the left and right of the li (0.75em and 2em respectively) the text can’t be centred since you’re forcing it off-centre with the padding.

If you amend the padding to: padding: 0.5em 1em; (0.5em top and bottom, 1em left and right) then it can be centred.

#nav-menu {
    font-family: Verdana, Arial, Helvetica, sans-serif;
    height: 30px;
    background-image: url(../img/menu_bg.png);
    background-repeat: repeat-x;
    border-bottom: dotted thin #666666;
    border-top: dotted thin #666666;
    text-align: center;
    width: 800px;
}

#nav-menu ul {
    list-style: none;
    padding: 0;
    margin: auto 0;
}

#nav-menu li {
    float: left;
    border-right: dotted thin #666666;
    list-style: none;
    padding: 0.5em 1em;
}

#nav-menu li a {
    line-height: 1.5em;
    color: #333333;
    text-decoration: none;
    font-size: 12px;
    font-weight: bold;
    display: block;
}

<div id="nav-menu">
    <ul>
        <li class="current_page_item"><a href="#" title="Home">Home</a></li>
        <li class="current_page_item"><a href="#" title="Home">Home</a></li>
        <li class="current_page_item"><a href="#" title="Home">Home</a></li>
        <li class="current_page_item"><a href="#" title="Home">zxczczxczHome</a></li>
    </ul>
</div>

JSFiddle demo of the above.