It seems that at least United Airlines lets you bring “medium-size” carry-on bags. By “medium-size”, I mean bigger than a backpack, but smaller than a suitcase. Instead of checking them in, bring them through security and to the gate. When you board, you can get a little green ticket, and pass the bag to some dude on the passageway to the plane. So you don’t actually bring the back with you to the cabin, but the nice thing is that you can pick up the bag right outside the door of the aircraft, rather than having to go to baggage claim and waiting nine hours for your bag to (not?) turn up. I don’t know if this applies to US domestic flights, all flights, or just United, but it’s a neat party trick.