I think this is pretty straight forward, though not as clean as it might be.
You can simply copy the category.php file in the Thematic theme folder to your child theme folder and rename it to category-catid.php where catid is the ID number of the category. For example, I have a custom template for one category, id no. 2013 in the database so my file is named category-2013.php and is only used for that category id.
In fact, with Wordpress 2.7 and above, when you are using a child theme, it looks into the child theme directory and overrides the parent themes templates with anything you have in there.
I hope this is helpful.
Dennis