I am in the process of getting an S&S coupled bike and have the same questions.

I am leaning towards the backpack case, although I understand that it still is somewhat cumbersome to pack and carry with you once you leave the airport so shipping to final destination may come in handy if practical. But you would need a box to put it in.
http://www.sandsmachine.com/ac_back.htm

Co-Motion thinks a hard case is not as good as you might think when comparing it against a soft case. For this reason, I think that a soft case like the backpack may be a good option. See their thoughts on the hard case.
http://www.co-motion.com/index.php/i...e_should_i_buy

Option two, as noted above you can use a disposable box and dispose of it. It is my understanding that a 26X26X10 box is needed.
http://www.staples.com/26-L-x-26-W-x...product_404572

The disposable box option won't work to get you home, for that maybe the option described by others to get a bike box and cut it to the right size and use a lot of packing tape. When I try to make a small box out of a big box, I have had the best luck putting folds in corrugated cardboard after I score it first with a pizza slicing wheel. Otherwise I have a lot of trouble getting it to fold where I want it to fold.

I find that a small electronic scale to check luggage weight is a very useful accessory. Less than $10 on ebay and shipped (slowly) from China. Airlines are very stringent on overweight luggage when it comes to fees.