//hook the administrative header output
add_action('admin_head', 'my_custom_logo');

function my_custom_logo() {
   echo '
      <style type="text/css">
         #header-logo { background-image: url('.get_bloginfo('template_directory').'/images/custom-logo.gif) !important; }
      </style>
   ';
}

http://www.smashingmagazine.com/2009/12/14/advanced-power-tips-for-wordpress-template-developers-reloaded/

Replace the default WordPress “W” logo in the administrative header with a custom one. Add this code inside the theme’s “functions.php” file.

Wordpress, Replace the default WordPress “W” logo in the dashboard

wordpress

PHP