I was surfing the net the other day and came across this page:
Benzworld DVD navigation retrofit
It seems that folks are now able to retrofit a dvd navigation system into their ML's at a cost of approximately $800-$1500 depending on where you source your parts. According to that thread in the link above its an easy upgrade.
Anybody have any idea as to what the part numbers would be for the BMW version which I'd imagine not to be that different than the MB one.