kubicka

If you don’t mind monkeying with code, you can do this:

Open template-functions-post.php in the wp-includes folder and find this:

function the_content($more_link_text = '(more...)', $stripteaser = 0, $more_file = '') {
$content = get_the_content($more_link_text, $stripteaser, $more_file);
$content = apply_filters('the_content', $content);
$content = str_replace(']]>', ']]>', $content);
echo $content;
}

then add this line just before the last line of the function:

$content = str_replace('<a href', '<a rel="nofollow" href', $content);

so the function now looks like this:

function the_content($more_link_text = '(more...)', $stripteaser = 0, $more_file = '') {
$content = get_the_content($more_link_text, $stripteaser, $more_file);
$content = apply_filters('the_content', $content);
$content = str_replace(']]>', ']]>', $content);
$content = str_replace('<a href', '<a rel="nofollow" href', $content);
echo $content;
}

Seems to work just fine.