I have just put an Aux in to my Cooper S with Harman Kardon,
It is a cheap fix that connects to the CD changer connector on the back of the head unit. I have removed my CD changer as you can't use both, but with a single CD in the dash already, I couldn't see me using the changer much with my iPod.
To find it just search "aux interface mini" on EBay. I paid ÂŁ29.99 delivered. (I can't post links yet)
This only provides an aux line in for a device, not any sort of integration for skipping tracks, but for the money, provides good sound quality for me without the expensive route of full iPod integration.
David