How do I set my custom input type as default?
In Drupal 7, the default input format is the one the users have the permission to use, and that appears first in the list of the input formats reported in admin/config/content/formats.



In my case, Textile is the default input format that is selected for the administrator role, but if the currently logged-in user doesn't have the permission to use that input format, then the default input format will be "Filtered HTML." "Plain text" is the input format that all the roles can access; in the case none of the other input formats is accessible to the user, "Plain text" is the input format that will be used. 



In Drupal 6, the default input format is the one selected as default in admin/settings/filters.