interesting idea. i don't think the ipod would be a good candidate, as i believe it's greatness comes from its simplicity, but i wonder if an idea like this would take off... having a compact computing unit that is pluggable to various shells.
my guess is that the current laptops would be hard to beat for the price/performance point of view. if this thing is to take off, it must offer at least the same functionality at about the same price. (or much greater functionality at not much increase in price.) novelty alone won't do it because there's only so much people are willing to pay to be able to email on the road or whatever. there's a reason people aren't really jumping on the camera phones and web-enabled phones in the u.s. their capability and usability is not justified by the additional price. (yes, it's a cool gadget, but again, novelty alone can't sustain anything for long.)
in addition, there's the concern for the computing unit security. as the prev. poster mentioned, now you can lose the most important part of your computer literally anywhere. with the wireless network rapidly spreading, i'd tend to think development of a "mobile" OS that can be accessed by dummy terminal shells anywhere would be more secure and logical..?