This misuse of the epxression drives me nuts :)
The following is an excerpt from:
I always use "I couldn't care less", or I would if I were the type of person who uses that expression much. I've been seeing "I could care less" very often recently, but that expression was practically nonexistent about five to ten years ago (or at least, I've never seen it around). It used to annoy me a lot (along with "could of" instead of "could've" or "ad nauseum" instead of "ad nauseam"), but since I've finished reading Mother Tongue by Bill Bryson, it doesn't annoy me as much anymore.
I don't think "I could care less" is going away anytime soon so we'll probably have to get used to it eventually. It might be illogical, but "the exception proves the rule" doesn't make much sense in contemporary English either ("prove" used to mean "test" = "the exception tests the rule"). So maybe we should all care a little bit less, or rather, we could care less (heh).