quicko: could care less

An Australian friend recently confessed that this Americanism had always gotten her (of course, she'd have said "got," as it's the standard British/Australian past participle of "get"; only Americans and possibly Canadians use "gotten"). She'd heard it in movies but was still convinced it ought to be "couldn't care less" -- which, I assured her, was true. If you're attempting to follow any semblance of logic, you should say, "couldn't care less" (which, for the record, is what I say). However, American English has somehow evolved to the point where either of these clearly contradictory statements means precisely the same thing -- frankly, we don't give a ... darn.