There are actually a few of them; the iTrip, a Monster one, a Dr. Bott one, a Gefen one (I believe), etc. You can get most of them through Apple's online store, CompUSA, or one of the Apple Stores. Of course, your friendly neighborhood VAR would appreciate a visit, too!

Unfortunately, most of these are specific to the iPod, so you'd probably need something more like Peter has to do this. (not to mention that they're fairly low powered)

I am the Doctor, and THIS... is my SPOON!