This is an old one, but still a great solution for busting browser cache any time you update your theme stylesheet. Mark Jaquith shows how to easily wrap your stylesheet with
filemtime, a PHP function that returns the time a file was last changed. That way, if the stylesheet loads and the file is unchanged, you don’t re-load it unnecessarily. But if the file has changed,
filemtime will force the browser to reload the stylesheet.
Bill Erickson also created a filter to do the same thing with Genesis themes.