I think the sarcasm (not irony) here has been absorbed into the phrase itself, so it no longer has to be spoken in a saracastic tone of voice. At least that's the only explanation I can suggest for its proliferation in the USA - I think it will always sound wrong to me compared with "couldn't care less".
"The Language Instinct"... that was the name of the book I read! Thank you for jogging my memory, seanahan.
I still respectfully disagree, though, on the basis of my own personal experience. Usually when I hear this phrase used, the user is unaware that there's any discrepancy between their intended meaning and their literal meaning.
Of course, we're talking about a bunch of different cases here; it may just be that you and Stephen Pinker hang out with a more erudite crowd than I do. :-)
As far as I can tell, this word is meant to be sarcastic. Steven Pinker says as much in "The Language Instinct", and it makes perfect sense. The tone with with which this term is spoken almost always makes the sarcasm completely clear.
I once read in a college textbook that this phrase is an example of irony. I don't believe it, though.
"Irony" implies that the speaker is consciously saying the opposite of what he/she intends to convey, and is hoping that the listener will hear the discrepancy. But "I could care less", though -- that's not conscious. People who say "I could care less" don't actually understand what they're saying, and you can verify this by asking them about it.
When I'm feeling charitable toward this phrase, I call it an idiom. When I'm not feeling charitable, I call it a mistake. Either way, I don't think it's a case of irony.