A black addon is now available for download.

Black version

Download the addon ( Installation instructions are included )

To begin open your single.php file.
If your theme does not have one then open your index.php file and save it as single.php.

Now save your single.php file as single-default.php
( this will become your default post template )

After thats done open single.php in code view and delete everything on the page replacing it with

post;
  if ( in_category('1') ) {
  include(TEMPLATEPATH . '/template-1.php');
  } elseif ( in_category('2') ) {
  include(TEMPLATEPATH . '/template-2.php');
  } else {
  include(TEMPLATEPATH . '/single-default.php');
  } ?>

In the example above all post in category 1 will be directed to template1.php , all post in category 2 will be directed to template-2.php and finally all others will use the single-default.php template.

You can add as many catagory templates as you like just be sure not to remove the last else statement for defaults.

Now open your single.php file again and save it as template-1.php or whatever you need to name it and modify it to fit the post category specs and repeat the process for each post category template you need.