I've just brought a xcarlink usb/mp3/sd adapter for my factory fitted radio/CD head unit.

fingers crossed it works! has anyone else fitted one?

this one: http://www.xcarlink.co.uk/product.php?productid=52&cat=13&page=1

Well I got it, and installed it!
Works great, filled a 4gb SD card with music, plugged in and great quality sound.
Also don't have to worry about keeping the ipod charged. Although the USB port will do that too!