Today is the 20th anniversary of the first smiley, according to Mike Jones. This has been widely blogged elsewhere, as you might expect, and there are dissenting voices, including those that point out that Plato had them first (Plato always seems to have had them first, no matter what "them" you're talking about).
Of course, smileys are just one example of the larger class of emoticons. :-)
Plato's emoticons are more interesting than ASCII ones, since they are formed by overstriking characters, rather than just stringing them together.
In any case, regardless of technology, or history, I find emoticons interesting for a few reasons. They remind me of our amazing ability to see faces everywhere we go (there's a passage in Scott McCloud's wonderful book Understanding Comics about this).
More importantly, they point out the need for us to have facial cues to understand language fully. There's an interesting theory of autism that believes that one problem autistics face is that they can't read the emotions on people's faces, and have therefore lost an important source of information about the people around them. One book on the topic is Teaching Children With Autism to Mind-Read, which sounds almost like a humourous title, but is not: it aims to teach autistics to understand more about the mental state of people (mind-reading) by looking at their faces.