Although this is a folding bike forum, I second the notion of looking into the Ritchey Breakaway. It's a full-sized bike that can be broken down to fit in a large suitcase. Also look into bikes equiped with S&S Bicycle Torque Coupler devices (
www.sandsmachine.com). These are large bolts which are installed in-line with the top and down tubes of a bike frame which allow the frame to broken in two parts, designed to fit into a suitcase. You can get frames pre-built (custom or production) or have an older steel or Ti frame retro-fitted with the couplers. What this gives you is NOT a folding bike that is ridable, it's a high-quality bike that happens to collapse for easy travel. I have ridden folding bikes and find them a great way to get around town, to/from work, on/off trains and buses, etc... And there are some very fine foldable frames out there. However, I find that the geometry and ride quality is just not the same as a "regular" loaded touring bike or a racing bike. Again, a bike with the coupler devices is not truly foldable. These are normal-looking bikes in which the frame can be separated into two parts for traveling. You can retro fit any style frame - touring, mountain, cross, racing... The whole thing, including wheels, fits into a suitcase which meets airline regulation - they don't know what's in it and don't charge extra. Also it's much easier to wheel around and take in a taxi than the huge full-size bike box. Check it out. I recently had a Surly Cross Check frame retro-fitted and will post when I get it completely built up.