Cache bust the style.css reference in WordPress.

functions.php

<?php

add_filter( 'stylesheet_uri', 'gmj_stylesheet_uri' );
/**
 * Cache bust the style.css reference.
 * 
 * Includes the file last modified time as a querystring.
 * 
 * @author Gary Jones
 * @link   https://gist.github.com/1892762
 *
 * @param string $stylesheet_uri Existing URI to style.css.
 * 
 * @return string Amended URI to style.css.
 */
function gmj_stylesheet_uri( $stylesheet_uri ) {
    return add_query_arg( 'v', filemtime( get_stylesheet_directory() . '/style.css' ), $stylesheet_uri );
}