From experience with friends in the business complain, complain, complain. Write letters, call, email... keep pressure on them.
Heard a story the other day about a customer who had a relatively minor problem, his car was in the shop for 3 weeks, he has a free loaner but he was not happy. He called every day!! His car is being declared a lemon and he is getting a new car!!
Here is a link to contact BMW USA.
http://www.bmwusa.com/BMW2003/Templa...T=Guest&bhcp=1
Start with the email. See what that gets you and if it does not help call and get an address to write to.
Oh, tell the service manager at the dealership you plan on writing BMW a nice long letter how you are not happy with how you have been treated. Tell him you plan on sending it to BMW USA, BMW Germany and whom ever else you can find an address for.
The dealerships are frowned upon by corporate for not taking care of their customers.
Good luck and do not give up!!! You deserve for your car not to have a silly leak. It is under warranty!!!!