"I could care less", meaning: I don't care already but I could care less as I am posting
"I couldn't care less", meaning: I absolutely don't care about this, but I cared enough to post on it
Notice how the first expression makes more sense as the second expression is ALWAYS hypocritical. How can you say you couldn't care less when you cared enough to post or read the original post to begin with.
I think, for the most part, both phrases are interchangeable because they both have the bottom line of this: "I DON'T CARE". No matter which phrase you use that is what you mean.
Also, I make the "your" mistake a lot, just because when I first started typing online I never bothered hitting the ' key. Now I hit it more often but for some reason I still pass it up when it comes to "your".
Oh yea, and the thing that bothers me most is people that use "u" instead of "you". Why? Because, in my experiences, people that type "u" are usually annoying to listen to. "U" is like a trigger for a bazillion typing errors, stupid hax0r speak, and other lame crap that follows.