>" I really liked the old American Airlines scheme where they just didn't paint the airplane."

It wasn't quite that simple. I believe the 'bare metal' finish actually had a layer of protective varnish painted on, to protect the metal from oxidation (anyone who has handled bare aluminum can tell you that it develops a layer of oxide very quickly). In addition to that, the panels had to be specially chosen and very carefully handled, as small blemishes were very obvious.