Display All Post Attachments and Assign Class to the Last Image?

This should do it, I think:

function get_images() {
  global $post;
  $attachment = get_children(array( 
      'post_parent' => $post->ID, 
      'post_type' => 'attachment', 
      'post_mime_type' => 'image', 
      'order' => 'ASC', 
      'numberposts'  => -1 ), 
    ARRAY_N );
  if ( $attachment ) {
    $attachment_count = count($attachment);
    foreach($i=0; $i < $attachment_count; $i++) {
      $last = ($i == ($attachment_count-1) ) ? ' last' : '';
      echo '<img src="' . wp_get_attachment_url($attachment[$i]->ID) . 
        '" class="post-attachment'.$last.'" />';
    }
  }
}