" I could care less" is sarcastic.
The sarcasm allows you to express more disdain than simply saying "I couldn't care less."
The expression "I couldn't care less" became common in England in World War Two and was apparently based on an earlier catchphrase "I couldn't agree with you more."
The "could" variant is first found in the mid-1960s and was being Objected to by the 1970s.
http://www.randomhouse.com/wotd/index.pperl?date=19960610
Righteous Robin!
