I just upgraded the Drupal engine to version 4.6.1. The upgrade process was cumbersome, awkward, non-trivial and difficult to say at least. It seems that Drupal suffers from the same upgrade disease as PostNuke does : the first time, the upgrade died on the database upgrade. Luckily, I did had a backup of the database and the website, so reverting the upgrade was no problem. I performed the upgrade then on a local copy, which learned me how to upgrade properly. There are some problems left, especially with the taxonomy_image module, but that's for later.