It was 25 years ago today; Dr. Fahlman taught e-text how to play.
On September 19, 1982, Professor Scott Falhman at Carnegie Mellon proposed :-) as a marker for jokes posted on a student-faculty electronic bulletin board. (Anyone besides me remember those?)
Since then, dozens, if not hundreds, of such "emoticons" have come into use.
Some purists decry emoticons. Emoticons, they say, contribute to the decay of language.
Balderdash. Throughout its history, written English has acquired new punctuation marks to help make meaning clear. The question mark (9th century?) and the exclamation point (as early as the 15th century!) are examples. More recently, so-called "scare" quotes have come into use, to signal that the word or phrase they enclose is being used ironically or otherwise unconventionally. (Scare quotes have, of course, been imported into spoken English as "air" quotes.)
As always, know your reader. Readers of formal documents (in fact, most documents printed on paper) will be bothered or distracted by an emoticon. But emoticons fill a real need in conversational e-mail, and they're here to stay.
So feel free to use emoticons. If your boss hassles you, better do what she says. But if anybody else hassles you, just refer them to me. I'll set them straight. :-)