BMW Portable Sat Nav is basically Garmin 720 with radio antenna for traffic information and installed in quite an ugly way instead of small glove compartment in the centre of dashboard. You can buy the same or better navi and put it on the windscreen - the result will be nearly the same, you will keep a storage, you will spend your money on something worthy
Try this thead:
http://e84.xbimmers.com/forums/showthread.php?t=360734