Aight...let's clear this shit up.
BOTH are acceptable.
"I couldn't care less" is the most obvious and accepted usage.
"I could care less" is an abbreviated form of "I could care less, but I'd have to try."
Granted, the speaker using the latter is usually misusing it, or oblivious to the proper phrasing, but for fuck's sake...let's just let this point of grammar fascism die already.