I’ve been thinking of weening myself off the Twitter, but Adium’s upcoming implementation looks ideal.
Fear not! Using Matt Gemmell’s MGTwitterEngine, a library used to communicate with the Twitter API, I’ve managed to make what I consider to be a very good Twitter client into Adium. Your friends (people you follow) are displayed in a group on the Contact List: adding or removing will follow or unfollow them, and chatting with them will send (or receive) direct messages. Note that you can only send a direct message to someone who follows you, so it might be a one-way conversation.
Read more about it on the Adium blog, including a screenshot of Adium treating Twitter basically like group chat.