That's great Nick,
However I think this is like shooting rabbits with a rocket. I think Arduino is OK for prototyping, but in the spirit of hacking something like DIY USB password generator | Code and Life or USB Business Card | Eleccelerator is more educational and not so hard to do as well for a fraction of the price.
Please don't get me wrong, I don't mean to lessen the achievement you have, just my recommendation.