Pioneer Car Audio CD/MP3/10GB HD Receiver

Ando and I talked about this (abstractly) yesterday. Interesting that Pioneer is opting to go with a 10GB HD in-car, versus simply supporting line-in from a portable MP3 player. I would much rather have a single portable hard drive than have disparate data stores in my car, portable player and home computer. Are line-in/out the solution? Bluetooth?