WpRecipes.com - Daily recipes to cook with WordPress

The first thing to do is to get your logo ready. Size might be 30*31px and the logo must be uploaded the images directory of your theme dir.
For exemple, if you're using the rockstar theme from WooThemes, your logo must be uploaded to wp-content/themes/rockstar/images.

Once done, open you functions.php file and paste the following code in it:

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>
   ';
}

Thanks to Smashing Magazine for this great hack!