I created a cck_taxonomy field and chose the vocabulary for it. After it's done, if I go back to the widget settings, the vocabulary selection is disabled.

line 26 in cck_taxonomy.module says:

        // Disable this since there will always only be one choice
        $form['widget']['widget_type']['#disabled'] = 'true';

The comment is definitely wrong, since I do have all vocabularies listed and not only one choice. Commenting this line out seems to fix this (I can change the vocab of the field).
Is there a reason for disabling the vocabulary selection?
I specified this as a support request, since I suspect it is by design..