I have seen a couple of Twentys that have been converted down to suitcase size. You are basically looking at taking a bare frame and replacing every component on it with alloy. You will have to separate the frame to get it to fit in a suitcase too. My preference for very frequent travel would be a Brompton, for less frequent travel a Bike Friday. Neither one is inexpensive, but based on what I spend on a typical upgrade and new parts you would come pretty close to the same amount of money, unless you have access to a lot of spare parts for cheap.

Don't get me wrong, I love my Twentys (all 3 of them) and do travel with them, but only by car. I also roll a 1984 Dahon Classic 3 that lives in the trunk of my company car, a Honda Accord. If I had to fly more it would be a Brompton 6 speed with the 12% reduction gearing.

Aaron