Brucie, I know you know the difference (flammable will burn, inflammable will explode) so you're asking about the semantics of the words, is that it?

Because of the confusion ("un," not "in," would mean it doesn't burn or explode), the Federal Highway folks here in the U.S. have for some years now stopped using "flammable" and "inflammable" and are using "flammable" and "explosive" on those signs (like these--scroll to the bottom and find links to the signs).

SRS