For those of you who are not aware (yet), I– and many other Mac users– am a frequent user of the Adium (or AdiumX) messaging client. Originally, I used it for AIM, but eventually branched out to the other supported services.
As Zac has mentioned, however, Twitter support will be making an appearance soon as an Adium feature, with the next major release: 1.4. This is apparently a frequently requested feature and, since the massive upsurge in tweeting among the Washington insider crowd and reporters, this will be a much desired capability.