I just wanted to give everyone a heads-up on this new product from USA SPEC. I just pre-ordered one from Crutchfield last week, and it's supposed to ship around April 8. It appears to operate in exactly the same way as the BMW solution (Five custom playlists correspond to Discs 1-5 on the head unit...Disc 6 is ALL songs). Plus it has an analog input for an additional audio source. Here's the link: USA SPEC iPod Interface
It's $119, plugs into the CD-changer port, keeps the iPod charged, etc..
I'll post a review after I receive it.