I was in a bike shop today and found a great folding bicycle: the Giant Halfway. It has 20" tires and is quick and easy to fold. Comes with its own bag and weighs 28 lbs. I rode it around the block and it was quite nice though I don’t think it would be good for anyone much over 6’ tall (I’m 6’ exactly and it fit surprisingly well).
http://www.giantbicycle.com/us/030.000.000/030.000.000.asp?section=8791&lYear=2002
The bike seems to cost about $600. The shop says that it is way better quality than the Dahon bikes that they sell (similar idea).
What are other airplane owners doing for bikes? Apparently the real hard core bikers get custom made frames with “S&S couplings” that let you take the main triangle apart. But with standard 26 or 27" wheels, the final result probably won’t be nearly as compact as the Giant Halfway (though you might save 10 lbs in weight).