Had to visit the dealer on Saturday and looking through one of the 'in-store' BMW Magazine (Summer 07) I came across an article on page 63 entitled:

The world is a disc. However it doesn't have to cost the Earth.

This gives info about trading in your old CD or DVD for a new 2007-2 disc. Prices are £65 for the CD and £99 for the DVD. It just says to benefit from the special trade-in, exchange the disc at your local BMW dealer.

Seems better than the £230 they usually charge.