Riga aggiunta.

HTML

<table class="list">
    <thead>
        <tr>
            <td><b>CON:</b></td>
            <td><b>OGG.</b></td>
            <td><b>N. MESS.</b></td>
            <td><b>ULTIMO MESS.</b></td>
        </tr>
    </thead>
    <tbody>
    </tbody>
</table>

JavaScript

$(function(){
    $("table.list > tbody").prepend("<tr><td colspan="4">Prova un altro</td></tr>");
    alert($("table.list > tbody").length);
});