Some clever guys in the US have devised something what they call a new punctuation mark (sic!). SarcMark is meant to “emphasize a sarcastic phrase, sentence or message”.
The mark is hardly going to have any success, but it has triggered a very interesting discussion on the LanguageLog (see comments for different ways of expressing sarcasm, link to a nice commentary by Geoff Nunberg and other interesting stuff).
I personally cannot help but ~admire~ the valiant breach of at least two communication principles: first, marking sarcasm so explicitly seems to be somewhat opposite to the idea of sarcasm; second, paying for a language unit/device is very opposite to the idea of language itself.