With the release of iPhone OS 3.0, I was wondering if emoji icons still worked (they do), and then it finally dawned on me: why not iChat? A quick search turned up exactly what I was looking for: Emoji for iChat. This free downloadable icon pack basically adds the iPhone Emoji icons to iChat, accessible from the standard smiley-face dropdown menu. For it to work, however, both sides have to have it installed; if your recipient does not use the tool, all they'll see is gibberish.
Emoji for iChat is a free download from Einar Andersson & Tor Rauden Källstigen. The download includes an installer which will require you to restart iChat.