Brompton's marketing of the B Bag make it sound like it's designed for flying with your bike.. If you add bubble wrap & clothes you're probably pretty safe, if you choose to check it in.

Based on my experiences flying to and from Japan (above), I think that I'll still gate check the bike next time. But I will try to get some clear plastic hose from the hardware store that is big enough to cover 1) the main frame tube, 2) the right fork blade, and 3) the handlebar riser. Those were the only places that got scratched last time. Using tough, clear plastic hose will not add any significant weight or bulk, and allow baggage handlers to see it's a bike - hence (in theory) they may still treat it a bit more carefully.