Language Log is a really cool site to visit. I was perusing the blog for some leads on a topic for this From the Field and I stumbled upon a hot, ongoing debate about "could care less" vs. "couldn't care less." A good place to start rummaging through this argument is Mark Liberman's entry, entitled "The Care Less Train Has Left the Station." Several familiar names have gotten in on this debate, including Arnold Zwicky and Steven Pinker.
For probably the hundredth time this semester, I've been reminded (thankfully) what linguistics is really about. My issue with the phrase was coming from some form of academic elitism, cloaked in a concern for "logic" (which is sort of silly, given that language is arbitrary). Steven Pinker has little concern for logic and instead argues that the prosody of "I could care less" is more conducive to conveying sarcasm. Mark Liberman disagrees, claiming the there is no standard of sarcasm in English, that there's nothing inherently sarcastic about the pronunciation that Pinker lists (I could CARE LESS), and further, that the prosody of that phrase isn't consistent in the first place. It's also an important not that an original definition does not equal it's most correct version. Further, for me personally, there are plenty of "illogical" phrases that go unchecked, such as "That'll teach you not to tease the alligators," vs. "That'll teach you to tease the alligators." I'm not really irked by either version, probably because someone I'd like to judge harshly hasn't uttered the "wrong" one since the discrepancy became salient.
Eric Bakovic checked out Richard Lederer's (Bakovic's public radio station co-host) claim that about half of the population uses either version of the phrase. Bakovic surveyed Google, finding that "Lederer's estimation jibes pretty well with Google. Searching for the strings "could care less", "couldn't care less", and "could not care less", I got 160,000, 131,000, and 19,200 ghits, respectively. Lumping the last two together, we get something pretty damn close to a 50-50 split."