I've got the Mazda MPS version of this:
http://www.xcarlink.co.uk/product.php?productid=66&cat=19&page=1" onclick="window.open(this.href);return false;
(They do Ipoo'd versions too)
It basically fools the head unit into thinking a CD Changer is attached. I just plug in a USB pen drive with 6 folders in it named CD01, CD02, CD03 etc (with as many files in each as you like in MP3 format) and you pick them on the head unit as you would a CD Changer.