It is my theory that a hard-to-see black cable hanging from your dash is less of an invitation for a break in.
I always take the iPod with me, but I leave the cable hanging. If you know what I mean. A white cable would scream both "iProduct" and "I like to spend a lot of money on gadgets. I may have left some in the car."
I have a black Gen 2 Nano, so they wouldn't see it, either, if not for the big white apple on the back.