If I'm not going directly to the office, I usually opt for jeans, tennis shoes (no metal), and a shirt with a pocket for my BP and/or passport.

The shoes are for comfort and (hopefully) avoiding shoe carnivals. The jeans can have anything spill on them or stain them and it doesn't bother me (too many filthy cabs).

I guess I also heed the warnings about folks who have been in emergency egress situations. I try to wear natural fibers and sturdy shoes. Not always long sleeves, because I think I've found all the planes where the a/c isn't cranked down to "Meat Locker".

I've done the suit and tie or business casual thing, but it seems like the clothes end up looking like I just sat in them for three hours (which I just did, heh).

I guess if somebody is trying to test a theory about class differences, I don't think there is a pattern. The suits tend to be salespeople, by and large, or more senior execs who can't get the corporate jet (or whose companies don't have them).