I have just invented a new form of punctuation. I know... it seems far-fetched, but really, inventing new forms of punctuation is not that hard. I'm filling a need here, just like Ron Popeil.
Anyway... in print, there are question marks to indicate queries, exclamation marks to indicate emphasis and periods to indicate a statement, but nothing lets a reader know that a sentence is meant to be sarcastic.
I am often online and I participate in several forms of written communication on the Internet like chats, bulletin boards and email. I'm sure that you do as well—since you're here reading this. I'm also sure you've seen a joke or sarcastic remark taken the wrong way because the reader doesn't regognize the author's intent. Why do we let this continue to happen? Is this a question? Yes, it is. You knew because of the question mark.
What I am proposing is a punctuation mark that clears up all confusion about sarcastic remarks for the reader. The closest thing to a sarcasm mark is the winking smiley—and he isn't really a professional tool. You can't write a missive to a business associate with little cutesy ASCII faces in it. It's just not done.