Kind of late to the discussion but just got my car and have been on a hunt myself.
There are several options. A 1 1/4" receiver made for the 2007 328i costs about $180 bucks from several folks online and made by Sportframe or Draw-Tite. They come with instructions and install without any drilling although a couple of appearance panels have to be removed and replaced. U-haul quoted me a Sportframe installed for $269.
I've had a receiver rack for years and in my book nothing works better for a bike rack, plus they give you the ability to put on storage rack or even pull a trailer of some kind if you had to, and yes, I wish it could be totally hidden, but it's a small price to pay. I love my bike too...