Lars Schröter's blog
Update: Our modifications to CiviCRM are in CiviCRM 3.0 core now. We will publish the Payment Processor and the corresponding Drupal module in the next weeks. Don't expect fast process though - we are quite busy currently. Use our demo at http://civi.fuerstnet.de (username and password: demo) meanwhile.
Update 2: Get a newer version of this text including downloadable files at http://fuerstnet.de/de/node/151 (german only).