I'm wondering why Americans say "I could care less" instead of "I couldn't care less" when surely the first statement is actually saying "I'm still mildly interested"?
I think it's actually because the phrase the Americans use was originally "AS IF I could care less". Quizmonster's example of our "cheap at half the price" is a very good comparison, as it is also used in a slightly miss-a-couple-of-words-out-and-therefore-make-it-a-bit-out-of-context-so-it-sounds-odd way: it's a costermonger's cry of (these goods are) cheap (today) at half the (usual) price.