Wordpress - Custom field values deleted when trashing custom post type

It seems that the action save_post gets fired when sending a post to the trash... Therefore, my custom metadata save function was being activated with no $_POST data to send thru it.

To circumvent this, I wound up adding a nonce to the save function.

function wpg_testimonial_author() {
  global $post;
  $custom = get_post_custom($post->ID);
  $testimonial_author_name = $custom['testimonial_author_name'][0];
  $testimonial_author_link = $custom['testimonial_author_link'][0];
  ?>
  <p>
    <label><?php _e("Author's Name:", 'quotable'); ?></label>
    <input name="testimonial_author_name" value="<?php echo $testimonial_author_name; ?>" />
  </p>
  <p>
    <label><?php _e('Attribution Link:', 'quotable'); ?></label>
    <input name="testimonial_author_link" value="<?php echo $testimonial_author_link; ?>" />
    <input type="hidden" name="testimonial_author_noncename" id="testimonial_author_noncename" value="<?php echo wp_create_nonce(plugin_basename(__FILE__).$post->ID); ?>" />
  </p>
  <?php
}

function wpg_save_testimonial_author($post_id) {
  global $post;
  if (!wp_verify_nonce($_POST['testimonial_author_noncename'], plugin_basename(__FILE__).$post->ID)) {
    return $post->ID;
  }
  if(defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) {
    return $post_id;
  }
  if(defined('DOING_AJAX')) {
    return;
  }
  if(!current_user_can('edit_post')) {
    return $post->ID;
  }
  if($post->post_type == 'revision') {
    return;
  }
  update_post_meta($post->ID, 'testimonial_author_name', $_POST['testimonial_author_name']);
  update_post_meta($post->ID, 'testimonial_author_link', $_POST['testimonial_author_link']);
}
add_action('save_post', 'wpg_save_testimonial_author');

Hopefully this might help someone else later...

Wow, thank you Nero_DCLXVI. Very helpful. For future people looking to implement this, here's simplified instructions of his solution:

1. Add this hidden input along with your other custom meta inputs:

<input 
    type="hidden" \
    name="prevent_delete_meta_movetotrash" 
    id="prevent_delete_meta_movetotrash" 
    value="<?php 
        echo wp_create_nonce( plugin_basename(__FILE__) . $post->ID ); 
    ?>" />

2. Add this right before your update_post_meta() functions.

if ( ! wp_verify_nonce( 
        $_POST['prevent_delete_meta_movetotrash'], 
        plugin_basename(__FILE__) . $post->ID ) 
    ) { 
    return $post_id; 
}