After blogging about the fact I was getting more than the regular comment-spams (which Dries verified in Mollom) I found out people were trying to abuse comments on my site because of the high PageRank. Even though the site is moderated and none of those comments saw daylight, I was fed up by the amount of moderation work.
Today I looked through the code to follow up on some commenters' advice to add rel="nofollow" attributes to my contributed links and I noticed that in include/theme.inc on line 1576 the following:
$output = l($object->name, $object->homepage, array('rel' => 'nofollow'));
should do what I want, only to discover that the line (according to function l()) should look like:
$output = l($object->name, $object->homepage, array('attributes' => array('rel' => 'nofollow')));
I reported it to Dries, he asked me to create this bugreport and forwarded it to Gábor Hojtsy.