Caring. Or Not.

Why do our transatlantic cousins say "I could care less" when surely what they mean is "I couldn't care less"?  Is there some subtle sarcasm going on here?  Or has the remainder of the sentence been snipped for brevity - maybe the full phrase would be "I could care less, but not very much less.."?

"I could care less".  So therefore you care more than the minimum amount possible, which tells me that you care.

I'm confused.  Maybe I should stop worrying about it.  Who cares?

Update - I googled and found this: http://www.antimoon.com/forum/2004/5458.htm . At least other people have wondered the same thing.

Update 2 - found this essay on the very topic: http://www.worldwidewords.org/qa/qa-ico1.htm. Key quote "people have tried to apply logic, and it has failed them" - well, that told me...