the chrome back would block the signal, making the iPod a unidirectional device, you'd have to point the screen in the direction of the other device to get a good signal. Something to think about.
Apple makes things user-friendly because they know which features to incorporate and which ones are too complicated. I think this is the latter.
The cards have to get thinner than a quarter before they could be incorporated without making the iPod larger. A card slot would make the iPod *signifigantly* thicker, remember a little plastic channel has to be inserted, for the card to slide into, and a card interface would need to be on a raised daughtercard, i'd guess it would add a quarter of an inch, at very least. This is why the batteries are sealed, because adding a battery compartment would have needed more space.
Perhaps a device that attached via the dock connector. Like an iTrip. The antenna would be outside of the iPod, and it wouldn't add space. It'd only be there when you need it. And it'd be available separately, so it wouldn't raise the iPod price unless you bought one.
paul