problemer med header

Jeg får denne warning når jeg er logget ind..

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/www/mydomain.com/include/switch.php:42) in /home/www/mydomain.com/include/session.php on line 46

Er der nogen der har en idé til hvad der kan være galt ?

Her er min switch.php

<?php
function category_content()
{
    $page = $_GET["page"];
    $content = "";

    switch($page)
    {
case 'details':
    include('details.php');
    break;

case 'userinfo':
    include('userinfo.php');
    break;

case 'useredit':
    include('useredit.php');
    break;

case 'result':
    include('processform.php');
    break;

case 'mybasket':
    include('basket.php');
    break;

case 'basketupdate':
    include('basketupdate.php');
    break;

default:
    include('frontpage.php');

    }

    $content .= "\n";

    return $content;
}
?>