I think sarcasm is best when unannounced. There will always be people who fail to understand it, and are forced to wonder whether someone is being serious, or just sarcastic. Perhaps they'll simply have to reread a post. Such a shame.

Smilies would take all the guess work out of it. And I, personally, enjoy making the reader struggle for comprehension sometimes.

*note the utter absence of smilies in this post*