I always thought "I could care less" was an Americanism. The civilised world says "I couldn't care less." But the Americans may have a point, depending on how you read it. "I couldn't care less." is generally taken to mean "My care factor is already at its lowest possible level. So, it would be impossible to care less." "I could care less." This COULD be taken as an ironic statement, saying "Hypothetically, it would be possible for me to care less, but in this world caring less would just be extra effort, so I'll be lazy and maintain my current level of care-lessness." Or, it could just be that Americans don't know what they're saying. Given their generally low capacity for irony, I suggest this is the likely explanation for most.