The patch is working quite nicely for me - in 12 weeks, only one spammer got through (and ended very quickly on my blacklist). So I extracted my code into a separate module, reducing the size of the patch for the original multiping module.
You can get it on my Drupal page. Don't forget: It is not enough to install the module, you have to apply the patch included to the original trackback module.

Ich habe vor rund acht Wochen das Trackback-Modul gepatcht, so daß es die sendende Seite einigen "Sanity Checks" unterzieht. Das ganze scheint tatsächlich zu funktionieren wie geplant: Nahezu der gesamte Trackback-Spam stammt von Botnetzen, weshalb die IP-Adresse des Trackback-Senders nichts mit der IP-Adresse zu tun hat, auf welche die im Trackback angegebene URL auflöst.

Since several people asked: Here is a patch (containing my trackback sanity checks) against the trackback module. Mind that this is just a draft, configuration of the behaviour has to be done in the source code. The preconfigured behaviour is to publish trackbacks which fulfill the sanity checks; all other trackback requests receive an error reply. You have been warned ;)

The update of the Multiping module for Drupal is now available on this site! I've been running it for three weeks now and it didn't cause any trouble, so I hope it will work fine on your site(s), too.
I've been experiencing some trackback spam floods recently, so I'm trying some new means of filtering:

• The server mentioned in the trackback link must resolve to an IP in the same subnet as the IP sending the trackback
• The page to which the trackback link refers must contain the URL of my site

Please help me testing this scheme: Write a blog entry with a link to my page, and send a trackback here. If the trackback doesn't get through, please drop a comment here. I'm curious whether this will work...

So, endlich habe ich es auch geschafft, die Seite auf die neueste Drupal-Version zu aktualisieren - bei der Gelegenheit habe ich auch das Layout überarbeitet (ich mag diese fixen Layouts nicht, daher habe ich das Zen Theme modifziert), in der Hoffnung, daß keiner der Browser damit Probleme hat :-)

Multiping

A replacement for the ping module of Drupal. It allows pinging of multiple sites (configurable via a web interface). If a site is unreachable, the ping request will be sent again during the next cron.php run.

Version 1.1 introduces several enhancements: It is possible to automatically select the pinged sites by the taxonomy terms assigned to the node. The sites can now be pinged during the cron run or immediately after a node submission. Finally, if a ping request failed, the retry time is increased exponentially in order to avoid "hammering" the service.