nvanhove [nvanhove]

1. Use <?php instead of <? for install file.
2. *Minor* Add
   /**
   * Implemenation of hook_foo().
   */
   to each hook you implement.

3.   global $blockdescription_allvalues;
     if(!$blockdescription_allvalues)
       $blockdescription_allvalues = blockdescription_get_all_values();
     
     if(isSet($blockdescription_allvalues[$module][$delta])) 
       return $blockdescription_allvalues[$module][$delta];
     else
       return false;
   
   

4. Add static instead of global.
5. use "{}" even for one-liner-ifs
6. isSet should be isset
7. false should be written as FALSE.

See the Drupal coding standards to understand how a module code should be written.

      db_query('DELETE FROM {blockdescription} WHERE module = "%s" AND delta = "%s"', $module, $delta);

Should be

      db_query("DELETE FROM {blockdescription} WHERE module = '%s' AND delta = '%s'", $module, $delta);

(and elsewhere) otherwise Postgres database users won't use your module

    $txt = "This module creates a description field so you can define what the purpose of the block is.";

t() ?

        $form[$form_element_id]['info']['#value'] .= "<br /><i>". $value ."</i>";

Why are you embedding html where themers can't get at it?

    foreach ((array)$form as $form_element_id => $form_element) {
      if (drupal_substr($form_element_id, 0, 1) == "#") continue;

element_child()?

Lastly, I'm pretty sure blocks are keyed by module/theme/delta where as you only use module/delta. Did you intend for your descriptions to cover all themes?