I stumbled over this too. In order to use the "selected" style for the tab in the standard MVC site.css file, you need to have the selected class be assigned to the <li> rather than the <a> tag.
ul#menu li.selected a
{
background-color: #fff;
color: #000000;
}
So to show the About item selected, for your menu use:
<ul id="menu">
<li><%= Html.ActionLink("Home", "Index", "Home")%></li>
<li><%= Html.ActionLink("Sites", "Index", "Sites")%></li>
<li class="selected"><%= Html.ActionLink("About", "About", "Home")%></li>
</ul>
