The Filter Default module allows you to assign a default input format for new nodes and comments for each role on your site. It adds a 'Defaults' tab to the Administer > Site configuration > Input formats page on which you can manage the assignments.
Drupal 6 users should take a look at Better Formats module, which covers more ground than Filter Default.
Note: This functionality will be built into Drupal 7, so Filter Default will not be maintained beyond Drupal 6.
Whenever a new node or comment is being created, each text input box with multiple allowed input formats is initially configured based on the settings for the user's roles. The user's lowest-weighted role (as defined on the settings page) that the user posseses is used. For a user that does not have any role indicated in the table, the default input format will be the one selected on the Input formats > List page.
This module has no effect on which input formats any user is allowed to use. It merely sets initially selected input format when there are multiple choices.
For example, suppose you have roles 'author' and 'admin' on your site in addition to the standard 'anonymous user' and 'authenticated user' and that your input formats defaults table looks like this:
Weight Role Default input format 1 admin PHP code 2 author Full HTML 3 4
Four rows are listed because you have four total roles and can assign a default input format to each one, but only two roles have assignments. For users with the 'admin' role, the default input format selected for any formatable text field will be 'PHP code'. For users with the 'author' role but not the 'admin' role, the default will be 'Full HTML'. For users with neither role, the default will be the site-wide default input format.
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