[EDIT] Totally editing this question. Custom fields do not work in my Drupal views, at all:
The interface to add the custom field worked just fine. It's only the generated sql thats wrong. Its refering to civicrm_contact.custom_3, but the value is actually stored in a related table called civicrm_value_[PRIVATE_PARTS_HIDDEN]_1.
What could be causing this ? I need a solution pretty urgent :-|
PS. I did map the values in settings.php, and this in general seems to work. One of the entries is
'civicrm_value_[PRIVATE_PARTS_HIDDEN]_1' => '[MY_CIVIDB_NAME_HERE].',
PPS. The checkbox used to add the field to the list, in the html source, reads
<input type="checkbox" id="edit-name-civicrm-contactcustom-3" name="name[civicrm_contact.custom_3]" value="civicrm_contact.custom_3" class="form-checkbox viewsImplicitFormSubmission-processed">