Let me preface this the best way I can, by quoting Wikipedia.
Though in the English language there is no standard accepted method to denote irony or sarcasm in written conversation, several forms of punctuation have been proposed. Among the oldest and frequently attested are the percontation point, - also known as an ironicon - invented by Henry Denham in the 1580s, and the irony mark, furthered by Alcanter de Brahm in the 19th century. Both of these marks were represented visually by a backwards question mark.
These various punctuation marks are primarily used to indicate that a sentence should be understood at a second level. A bracketed exclamation point and/or question mark as well as scare quotes are also sometimes used to express irony or sarcasm.
[sarcasm]Sarcasm tags are a perfect solution to the problem. Mostly because there's no one in the entire world who doesn't understand basic programming.[/sarcasm]
Seriously though, in this day and age where huge numbers of people are communicating via written text for hours at a time, how is it that we have not globally recognized the need for an "ironicon"?