Okay, apparently you can plug an HD in via 30pin-to-USB, but there's no software yet to recognize it. So in theory a media player could do it, other than iTunes. I'm looking for an app like that for a jailbroken iPhone, so far, no luck. I'll keep looking.
(I've seen tons of people trying to turn the iphone itself into an external HD...am I missing something or that's just plain stupid? Why? external HD are dirty cheap...)