Im using thickbox and this doesn't work for me. This is why I'm asking about the "core" layout.
What you're looking for doesn't exist yet. It wasn't possible to ship .tpl.php files with modules in Drupal 5, but it is in Drupal 6. This is one thing we plan to update during Drupal 6 development.
awesomeness!! guess it's starting to be about time that I update to D6 :$