http://www.dillerdesign.com/experiment/DD_belatedPNG/
Here is an example: <!--[if IE 6]>
<script src="DD_belatedPNG.js"></script>
<script>
/* EXAMPLE */
DD_belatedPNG.fix('.png_bg');
/* string argument can be any CSS selector */
/* .png_bg example is unnecessary */
/* change it to what suits you! */
</script>
<![endif]-->