Thank you Wikipedia!

I've always wondered about this. Now I know -

http://en.wikipedia.org/wiki/Flammability#Linguistics:_flammable_vs._inflammable

Flammable and inflammable both mean capable of burning. The word "inflammable" came from Latin inflammāre = "to set fire to," where the prefix "in-" means "in" as in "indoctrinate", rather than "not" as in "invisible" and "ineligible". Nonetheless, "inflammable" is often erroneously thought to mean "non-flammable". This safety hazard has been avoided by the use of flammable on warning labels referring to physical combustibility. In the United States the word inflammable has been largely abandoned in common, scientific, industrial, and written language.^[1] Antonyms of flammable/inflammable are non-flammable, non-inflammable, incombustible, non-combustible, uninflammable, or simply "not flammable".