I've ignored the airline regs and take my Dahon MU SL in a cheap 32" case. I can fit it in the case using its normal fold with the wheels on (tyres let down) and the seat post out.
I haven't been refused yet, but then I haven't taken it out of europe
The dahon airporter case is designed to take on a plane but costs a fortune and doesnt' comply with the size constraints...
One thing to bear in mind is the weight of the case. You'll need the case and the bike to be below 20kg to avoid charges for excess baggage. My case and bike are light enough so that I can also throw some of my gear and spares into the case as well.