I never had a leaky bimmer, but I had a friend who did have a problem in his 325i, he thought it was the weather strip, but after a few trys it turned out to be a very slight dent along the weather stripping, he finally got it fixed. not sure how much it cost him. by the way I've been shopping around for a new a car and I found an offer where I could get a free subscription to media week magazine for test driving the Lexus IS sport design. you mught want to check out these links.
http://www.mediaweek.com/mediaweek/index.jsp
http://www.adweek.com/aw/index.jsp
http://www.brandweek.com/brandweek/index.jsp