Hello,
I've already located the problem, and for someone a bit more clever than me, it should probably be pretty easy to correct this problem - at least that's what I'm hoping :)
If I remove the following code, the plugin rewrites the titles without a problem, but if I leave this code be, all posts and pages will have the title of this php file.
What the about.php actually is, is a small preview of the about me page, shown just above the widget sidebar.
<!--about-->
<div id="about">
<?php query_posts('pagename=om mig'); ?>
<?php while (have_posts()) : the_post();
$myphoto = get_post_meta($post->ID, "myphoto", TRUE);
$mydesc = get_post_meta($post->ID, "mydesc", TRUE); ?>
<h3>Om mig</h3>
<a href="<?php bloginfo('url'); ?>/om-mig"><img src="<?php echo $myphoto; ?>" alt="<?php bloginfo('blogname'); ?>" title="<?php bloginfo('blogname'); ?>" /></a><span><?php echo $mydesc; ?> ... <a href="<?php bloginfo('url'); ?>/om-mig">mere</a></span>
<?php endwhile; ?>
</div><!--about-end-->