Strip inline width styles from WordPress's image caption shortcodes to allow for responsive images.

remove-inline-image-widths.php

//* Strip width from image captions to allow responsive code to shrink images.
add_shortcode('wp_caption', 'fixed_img_caption_shortcode');
add_shortcode('caption', 'fixed_img_caption_shortcode');
function fixed_img_caption_shortcode($attr, $content = null) {
  if ( ! isset( $attr['caption'] ) ) {
    if ( preg_match( '#((?:<a [^>]+>\s*)?<img [^>]+>(?:\s*</a>)?)(.*)#is',
      $content, $matches ) ) {
      $content = $matches[1];
      $attr['caption'] = trim( $matches[2] );
    }
  }
  $output = apply_filters('img_caption_shortcode', '', $attr, $content);
  if ( $output != '' )
    return $output;
  extract(shortcode_atts(array(
    'id'	=> '',
    'align'	=> 'alignnone',
    'width'	=> '',
    'caption' => ''
  ), $attr));
  if ( 1 > (int) $width || empty($caption) )
    return $content;
  if ( $id ) $id = 'id="' . esc_attr($id) . '" ';
    return '<div ' . $id . 'class="wp-caption ' . esc_attr($align) . '" >' . do_shortcode( $content ) . '<p class="wp-caption-text">' . $caption . '</p></div>';
}