Problem with using portable hard drives becomes powering them and securing them... which pretty much overrides the idea of "not using an ipod."
Seagate had a great idea with D.A.V.E. a solid slate disk powered by a battery with bluetooth and wi-fi interface. However, for some reason that project never got off the ground. If it did, I would throw one of those under the passenger seat.... and forget about it.
I am sure pretty soon we will start seeing decks that will take advantage of SD cards that have bluetooth and wireless USB (WUSB) standards, which will solve such issues fairly well.
Yeah, that deck is expensive
-Sov13t.