I just stumbled across this on Mark Jaquith’s blog.
It’s a really handy way of getting around the automatic caching that happens to css files.
<link rel="stylesheet" href="<?php bloginfo('stylesheet_url'); echo '?' . filemtime( get_stylesheet_directory() . '/style.css'); ?>" type="text/css" media="screen, projection"
Basically it uses the timestamp as the variable that changes. Anytime you update the css file any browser will think it’s a new file and go and get it.
Simple and useful - thanks Mark!