Correctly implementing the Facebook Like button

Yoast.com actually covered this quite recently…

if (  is_singular() ) {
  global $post;
  if ( current_theme_supports('post-thumbnails') 
      && has_post_thumbnail( $post->ID ) ) {
    $thumbnail = wp_get_attachment_image_src( 
      get_post_thumbnail_id( $post->ID ), 'thumbnail', false);
    echo '<meta property="og:image" 
      content="'.$thumbnail[0].'" />';  
  }
  echo '<meta property="og:title" 
    content="'.get_the_title().'" />';
  if ( get_the_excerpt() != '' )
    echo '<meta property="og:description" 
      content="'.strip_tags( get_the_excerpt() ).'" />';
}

Explanation and more details are here: http://yoast.com/facebook-share-wordpress/