A couple of notes..
How the Japanese let their hair down can easily we seen in a Wii commercial:
http://www.youtube.com/watch?v=YiWuTopIrxk
The general idea is that if it can be bought at the GAP you're good. And japanese school children wear their uniforms for everything, including some serious parkour:
http://www.youtube.com/watch?v=WfKxpmnAFRw
That said, the dressing-down of airlines directly relates to the dressing-down of a visit to a Disney theme park. What happened? The 70's.
Disney originally denied improperly dressed hippies from entering Disneyland. Even being a male with long hair kept you out. Social norms changed and Disneyland let them in. The same likely happened in air travel.