Custom theme and preprocessing

I'm creating a new look for the revision summary for a node, but I can't seem to get the preprocessing for the theme to work as described in various literature. My override is being called, but the preprocessing function that I want to change some of the row values is not.

Right now, I have this code:

/**
 * Implementation of hook_theme().
 * Register the theme_hooks() available in this module, with their arguments
 * and default values.
 * 
 * This is working.
 */
function gravityswitch_theme() {
  return array(
    'table_revisions' => array(
      'arguments' => array('header' => NULL, 'rows' => NULL , 'attributes' => NULL, 'caption' => NULL),
      //'template' => 'revisions-table', // revisions-table.tpl.php
    ),
  );
}

/**
 * We have to do some stuff to the table for revisions before passing it on, like adding the extra link to the cells
 *
 * Currently, it should just echo some variables and stop to prove that it gets there,'
 * But it doesn't.
 * 
 **/ 

function gravityswitch_preprocess_table_revisions(&$variables) {
    echo("variables:<br />\n");
    print_r($variables);
    exit;
}

/**
 * Copied from the table theming code to start with.
 * 
 * Implement (in your own module) the function below if you want to override
 * the way in which the Revisions table is constructed.
 * If you do, don't forget to register this theme_hook() via <your_module>_theme()
 * in a manner similar to revisioning_theme() in this file.
 * 
 * @param $header
 * @param $rows
 * @return themed HTML, see for instance /includes/theme.inc/theme_table() and
 *         diff.module/theme_diff_table()
 *
 * @ingroup themeable
 * 
 */
 
 function theme_table_revisions($header, $rows) {
     // Add sticky headers, if applicable.
     .
     .
     .
     return $output;
}

If anyone can see what I'm doing incorrectly, I would be very grateful.