I agree with Mark it's actually less work to just keep a separate set of winter wheels/tires. It's too much of a hassel to mount/unmount tires 2x a year. But, if u did choose to go the route of mounting/unmounting, don't go to your dealer, just go to a local wheel/tire place and they can do it for cheaper probably (just make sure they have good soft mount machines that can mount 17" and 18" wheels without scratching them).
p.s. you don't have to get ugly steel wheels if you want to get dedicated winter wheels to use with your winter tires. there's a sticky at the top of this forum for winter wheel+tire sets... here it is
http://www.e90post.com/forums/showthread.php?t=631