Here's another little error you hear on a daily basis in the States, and one that can really bug uptight language nerds like myself. Sadly, it's probably not going anywhere any time soon, but you can still do what you can by correcting your friends and family next time you hear it!
These phrases at first might appear interchangeable - they both mean the same thing, right?
No such luck. Unless you really are trying to say that you could care less about something - and that can pretty much be descriptive of any level of caring - you should only technically use the first one. If you cared about something a lot, you could easily care less about it. But then, if you hardly cared about something at all, you could still care less.
On the other hand, if you couldn't care less about something, that means that your current level of caring is zero. Zilch. Nil. There is no way that you could care less about it, and that's what you're emphasizing. You care so little about it, that you couldn't possibly give less of a hoot.
Therefore, "couldn't care less" is correct; "could care less" is incorrect.
Shamefully stolen from incompetech.com, this little graph sums up the oft-mistaken concept quite nicely:
So there you have it. "I could care less" really means nothing, if you think about it.