I had an idea, so bear with me- I was thinking that a BMX bicycle, with an extended seat tube and extended steerer to give reasonable height, just might work as a "folding" portable bike. Now obviously, it wouldn't actually fold, but a mid-size BMX frame might (?) meet the requirements for bringing it on a bus, train or aeroplane and of course it has the small 20" or maybe even 16" wheels of a folder. So for the "folding" (actually stowing) action, I was thinking-
Using an allen key and a quick release front wheel and seatpost- loosen the steering stem, drop the bars and flip them to be horizontal. Loosen the seat post, drop it as far as possible. Release the front wheel, and using a cable lock, tie it to the back one.
It's conceptual, but if it works it would be a good way to get a very cheap stowable bicycle. All you would need is a 16 or 20 inch BMX bike (on craislist or garage sales all the time), an extended steering stem and seat tube (on ebay or LBSes) and a light for the front, and there you have it, a ghetto folder.
Any problems you can forsee with this? Will the average BMW frame just be too long without folding, or maybe there's better type of frame out there such as an old steel kid's bike frame (minus the "princess" or "rocketship" decals of course )