i've taken my bike in a cardboard box overseas (la-->denmark) twice. the first time over, no problems except the box was huge and weighed a ton. coming back, i cut the box down a bit and ran out of tape. let's just say it was a horror fest when my bike came out of the oversized baggage area in a complete mess. the whole box was decapitated and falling apart. this was all due to my poor boxing but the shock aside, everything was totally fine. the woman even picked up a pedal off the ground that slid out. that was such a scary moment since i thought my bike was totally fubarred.

so aside from being ghetto and potentially hazardous, cardboard boxes are fine/free if you pack your bike properly. i didn't get charged on british airways, it was counted towards my luggage allowance and swiss air said they'd charge. but the woman at the check in counter didn't charge me. i think it was because i had a "reservation" which consisted of me calling in to say i needed to reserve room on the flight for my bike. as for domestic, i haven't had to do it yet. maybe you could call them and try to get it for free if you aren't checking in any other luggage? be polite and insistent and you might be able to squeeze a deal.