I think most folks put their bikes in a hard suitcase to protect it.
Then there are those folks that are trying to beat the fees that
most US carriers charge for flying with a bike. Check with your
specific airline. I went from Newark Liberty(NJ) to China(Shanghai)
this summer(2012), I packed my folding bike in my sole check-in
luggage:

http://www.youtube.com/watch?v=eUQNU2DKiWw