I know this is an old post so sorry about re-opening it, however I just had a similar problem on my X6 (only 6 weeks old!!).
I think the corrosion has came from the amount of salt on our roads (we have had snow lying since November), however I am into car cleaning in a big way and clean my car weekly, so think it is fair to say that the finish is not fit for purpose.
Rather than waste my time at the dealer, I decided to try and fix this myself.
The good news is that after 1/2 hour polishing with very fine wire wool (00 stuff, see
http://www.cleanyourcar.co.uk/access.../prod_377.html) I successfully removed all the surface corrosion and then polished / protected with a metal polish (
http://www.cleanyourcar.co.uk/engine.../prod_780.html) and it looks like new again