According to the BMWNA website:
"The Disposition Fee is a charge included on your Lease Agreement, which is used to defray the cost of preparing and selling the vehicle at the end of the lease (either scheduled or early termination) if the vehicle is returned. It is not due if you purchase the vehicle. As a courtesy to our returning customers, this fee will be credited to your next account if you finance with BMW Financial Services within six (6) months."
It does not appear to be refundable if you buy a DIFFERENT BMW, but if anyone knows about this, I would be interested to hear.