Of course we should have a sarcasm punctuation mark. That would help people to better appreciate it.
Okay, now that I'm done with the necessarily sarcastic remark, I really think that we shouldn't have a mark for it. Yeah, on the internet, it can be nigh impossible to tell if a person is being sarcastic or not if they just type out some text (especially if the thread is about sarcasm, "Are they being sarcastic or truthful? Sarcastic or truthful? Damn this is annoying!") but part of why I like sarcasm so much is that its subtle. Its easy for people to take you at face value when you're being sarcastic. They look at you weirdly, wondering why you just said what you did, stop, then realize it was sarcasm. Of course if they don't and just continue to take your words as what they seem to be, they probably wouldn't appreciate sarcasm anyway. Its like in-jokes, if you know it you'll laugh, but it shouldn't be spelled out to those who didn't get it. Don't explain the joke [http://tvtropes.org/pmwiki/pmwiki.php/Main/Ptitle0t9r68ih?from=Main.Dontexplainthejoke].
Sometimes you may want to use a mark to show you're words aren't to be taken too seriously, but you shouldn't be forced to. Then people would have even more trouble picking it up when the mark wasn't used.